Analyst View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
PAPER
View Journal | View Issue
Ionic strength assay via polyacrylate–ferriferrous oxide magnetic photonic crystals†
Cite this: Analyst, 2015, 140, 3368
Yan-Ran Li, Ye Sun and He-Fang Wang* Convenient reading out and/or determination of ionic strength (IS) is of great significance for both scientific research and real life applications. We presented here a novel method for the rapid and sensitive IS assay based on the electrolyte-induced sensitive wavelength blueshifts of the reflection spectra of polyacrylate capped Fe3O4 magnetic photonic crystals (PA–Fe3O4-MPCs). For HCl, MgSO4 and the common electrolytes corresponding to the salinity of seawater (including NaCl, KCl, MgCl2, CaCl2, Na2SO4 and their mixtures), the PA–Fe3O4-MPCs displayed wavelength blueshifts identical to the total IS of the aqueous solutions, regardless of the kind of above-mentioned electrolytes in the solutions. Besides, the PA–Fe3O4-MPCs exhibited relatively high sensitivity (an average of 294 nm L mmol−1 in the range of 0.05–0.30 mmol L−1, and an even higher value of 386 nm L mmol−1 at 0.05–0.15 mmol L−1) and fast response (within 8 s) to the IS of aqueous solutions. The relative standard deviation (RSD) for IS (NaCl, 0.1 mmol L−1) was 4.4% (n = 5). The developed method was applied to determine the salinity of seawater
Received 2nd February 2015, Accepted 20th March 2015
samples, and the determined results were validated by the traditional standard chlorinity titration and
DOI: 10.1039/c5an00218d
electric conductimetry method. The recoveries were in the range of 92–104%. The proposed PA–Fe3O4MPCs based reflectometry method would have great potential for IS and salinity assays.
www.rsc.org/analyst
1.
Introduction
The ionic strength (IS) of a solution is a function of the concentration of all ions in that solution, calculated as: I¼
n 1X ci zi 2 2 i¼1
ð1Þ
where ci is the concentration of ion i (often in M, mol L−1) and zi is the charge number of that ion.1 IS plays an important role in Debye–Hückel theory and double-layer theory and relates to electrokinetic and electroacoustic phenomena in colloids and other heterogeneous systems.2 Besides, IS monitoring is important for natural aquatic ecosystems, climate models and chemical oceanography.2 Consequently, convenient reading out and/or determination of IS is of great significance for both scientific research and real life application. To date, several methods based on different principles have been used for the measurement of IS (or salinity), including traditional chlorinity titration, electric conductimetry,2 fiber Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin Key laboratory of Biosensing and Molecular Recognition (Nankai University), State Key Laboratory of Medicinal Chemical Biology (Nankai University), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 94 Weijin Road, Tianjin 300071, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5an00218d
3368 | Analyst, 2015, 140, 3368–3374
optical sensors,3 photonic crystal fibers,4 displacement/differential measurements,5 interferometry,6 fiber resonators7 and photonic crystal reflectometry.8 Every method has its pros and cons; photonic crystal reflectometry has the merits of nakedeye inspection and is thus promising for convenient monitoring and detection of IS (or salinity).8 However, only a few papers on photonic crystal reflectometry have been published, where poly(styrene-co-sodium styrenesulfonate) templated polyacrylamide hydrogel was used as the sensing material.8 Photonic crystals (PCs), consisting of periodical structures of different dielectric materials, were first reported by Yablonovitch9 and John.10 The most noteworthy feature of PCs is their colorful appearance,11–13 which arises from interference and reflection according to the Bragg’s and Snell’s laws:14 λ ¼ 2Dðneff 2 sin2 θÞ1=2
ð2Þ
where λ is the wavelength of the reflected light, D is the distance of diffracting plane spacing, neff is the average refractive index of the PCs, and θ is the angle of incident light. The dependence of λ on the features of PC materials (e.g., D, neff, and θ) enables the design and application of various PC sensors. Recently, PCs have been used for sensing various external stimuli15,16 such as pressure,15 coexistence substances,17–24 temperature,25,26 humidity,27–29 pH,30,31 and ionic strength.26,31 PCs used were mainly fabricated by nanolithograph and
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Analyst
colloidal assembly methods,32 therein colloidal assembly was usually assisted by gravity, electricity and magnetism.33 For real application of colloidal assembled PCs, the solidification of periodic structure was always required. However, the solidification process, usually involving the use of cross-linking monomers, initiators and other additives, might result in the disassembly of the periodic structure of PCs.26 Magnetic PCs, assembled via the superparamagnetic colloidal Fe3O4,34,35 have fast-assembly and solidification-free brilliant structural colors. Besides, the colloidal Fe3O4 can be easily prepared by the solvothermal method, and the resulting Fe3O4 can be homogeneously dispersed in some solvents and magnetically self-assembled to PCs, thanks to the balance of electrostatic repulsive force36/steric repulsion37 and magnetic force. The Fe3O4 PCs have been pioneered by Yin et al.36,38 but the real application of Fe3O4 PCs for optical sensing of IS has not been reported so far. Herein, we represented a novel method for the rapid and sensitive determination of IS based on the electrolytesinduced sensitive wavelength shifts of the reflection spectra of polyacrylate capped Fe3O4 magnetic PCs (PA–Fe3O4-MPCs). The HCl, NaOH, MgSO4 and the most common electrolytes corresponding to the salinity of seawater39 (including sodium chloride, potassium chloride, sodium sulfate, magnesium chloride, calcium chloride and their mixtures at different proportions), were involved in evaluating the feasibility of this IS sensor. Interestingly, the PA–Fe3O4-MPCs displayed wavelength-shifts almost identical to the total IS of the solutions composed of HCl, MgSO4 and/or the electrolytes corresponding to the salinity of seawater. Besides, the PA–Fe3O4MPCs sensor displayed rapid and sensitive response to the IS of aqueous solution. Consequently, the developed PA–Fe3O4MPCs sensor would have great potential for IS and salinity assays.
2. Experimental 2.1.
Reagents
All reagents (at least of analytical grade) are used directly. Iron(III) chloride hexahydrate (FeCl3·6H2O, 97%, Aladdin, Shanghai, China), anhydrous sodium acetate (99%, Aladdin, Shanghai, China), sodium acrylate (99%, Heowns, Tianjin, China) and ethylene glycol (EG, Aladdin, Shanghai, China) were used for preparing polyacrylate capped Fe3O4 (PA–Fe3O4) nanoparticles. The electrolytes including sodium chloride, potassium chloride, sodium sulfate, magnesium chloride, calcium chloride and magnesium sulfate were all purchased from Guangfu fine Chemical Research Institute (Tianjin, China). Silver nitrate (99%, Aladdin, Shanghai, China) was used for titration of chloride ions and potassium dichromate (Guangfu fine Chemical Research Institute, Tianjin, China) was used as the indicator for monitoring the titration end point. All solutions were newly prepared in ultrapure water (Wahaha, Hangzhou, China). The standard NdFeB magnet was used throughout all the experiments.
This journal is © The Royal Society of Chemistry 2015
Paper
2.2.
Apparatus
The morphology of PA–Fe3O4 nanoparticles was observed on a JEOL 100 CXII transmission electron microscope (TEM) operating at a 200 kV accelerating voltage. The samples for TEM were prepared by drying sample droplets from ultrapure water dispersion onto a 200-mesh Cu grid coated with a lacey carbon film (Zhongjingkeyi Technology, Beijing, China). The X-ray diffraction (XRD) spectra were recorded with a Rigaku D/max2500 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5418 Å). The Fourier transform infrared (FT-IR) spectra (4000–400 cm−1) in KBr were recorded with a Nicolet MAGNA-560 FTIR spectrometer (Nicolet, Madison, WI). The magnetic properties (M-H curve) were studied using a LDJ 9600-1 vibrating sample magnetometer (LDJ Electronics Inc., Troy, MI, USA) at room temperature by cycling the field from −6 to 6 kOe. The thermogravimetric analysis (TGA) was conducted with a PTC-10A TG-DTA thermo-analyzer (Rigaku, Japan). The Zeta potentials and dynamic light scattering (DLS) were measured on a laser light scattering spectrometer Zetasizer Nano ZS (Malvern, UK) at 25 °C. The reflection spectra of the PCs were measured on an HR4000CG-UV-VIS-NIR spectrometer (Ocean Optics, FL, US). The DDSJ-308A conductivity meter (YiChen Technologies, Xiamen, China) was used for determination of the salinity of seawater samples. 2.3.
Synthesis of PA–Fe3O4 nanoparticles
The PA–Fe3O4 nanoparticles were synthesized according to the literature.40,41 In a 100 mL flask, FeCl3·6H2O (0.54 g) and EG (20 mL) were mixed by magnetic stirring to obtain a homogeneous yellow solution, then sodium acrylate (0.3 g) and CH3COONa (1.2 g) were added successively and kept stirring for 1.5 h. The resulting viscous yellow solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h, and then the autoclave was cooled to room temperature. Finally, the black products were washed with 40 mL of pure water–ethanol (1/1, V/V), 30 mL of pure water and 30 mL of ethanol respectively, and the resulting solid was dried under vacuum overnight. 2.4.
Reflection measurements
All measurements were carried out on an HR4000CG-UV-VIS-NIR spectrometer equipped with a reflection probe. Typically, the reflection probe was placed perpendicular to the glass vessel containing the aqueous PA–Fe3O4 nanoparticles solution (1 mL, the solution was subject to ultrasound for 5 min before use), and the NdFeB magnet was placed in the direction opposite to the reflection probe (in the other side of the glass vessel). For optimization of the measurement conditions, the NdFeB magnet was placed at different distances (5.0–1.5 cm) from the 1 mL solution of PA–Fe3O4 nanoparticles (with various concentrations of 2.0–18.0 g L−1). For the IS assay, the concentration of PA–Fe3O4 nanoparticles solution was fixed at 4.0 g L−1 and the distance between the NdFeB magnet and PA–Fe3O4 nanoparticles solution was fixed at 3 cm. Different electrolyte solutions (IS 10 mmol L−1, 5 μL each time) were
Analyst, 2015, 140, 3368–3374 | 3369
View Article Online
Paper
Analyst
added into the PA–Fe3O4-MPCs solution (1 mL, 4.0 g L−1) and stirred gently by the pipette tip for 2 s, and then the corresponding reflection spectrum was recorded 6 s later. The reflection measurements were done at 25 °C, or as otherwise stated.
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
2.5.
Real samples analysis
The samples were collected from different regions of the Bohai Sea (Tianjin, China). All samples were filtered through 0.45 μm Supor filter and stored in the precleaned glass bottles at 4 °C fridge. As the average salinity in the world’s ocean is as high as 3.5% (35 g L−1, or 599 mM), all the samples were diluted 20 times before determination. Typically, 2 μL of the diluted seawater sample was added into 1 mL of PA–Fe3O4 MPCs (4.0 g L−1) solution and the reflection spectra were measured by the above-mentioned procedure. For validation of the proposed method, 0.5 mol L−1 of NaCl was spiked in the filtered seawater samples, and 2 μL of the 20-time diluted spike-samples were added into 1 mL of PA–Fe3O4 MPCs (4.0 g L−1) solution to determine the recoveries. For verification, the salinity of the seawater samples was also determined by the standard electric conductimetry method and chlorinity titration, where the filtered seawater samples were 20-time diluted before determination.
Fig. 1 (a) XRD pattern, (b) FTIR spectrum, (c) TGA curve, (d) magnetic hysteresis curve, and (e) TEM of the PA–Fe3O4 nanoparticles, and (f ) digital photos of the PA–Fe3O4-MPCs with 4 g L−1 of PA–Fe3O4 nanoparticles assembled under different magnet intensities (by changing the distance between PA–Fe3O4 nanoparticles solution and NdFeB magnet, 5 to 1.5 cm, left to right).
3. Results and discussion 3.1. Characterization of PA–Fe3O4 nanoparticles and PA–Fe3O4-MPCs The prepared PA–Fe3O4 nanoparticles displayed typical face center cubic phase of Fe3O4 (JCPDS no. 19-0629) as shown in the XRD pattern (Fig. 1a). The strong peak at 580 cm−1 in the FTIR spectrum (Fig. 1b) was the hallmark of Fe–O stretching vibration, also demonstrating the existence of Fe3O4. The peaks around 1559 and 1410 cm−1 (Fig. 1b) corresponding to the COO− asymmetrical and symmetric stretching vibration indicated the presence of a polyacrylate coating. The existence of a polyacrylate coating was further ascertained by the TGA curve (Fig. 1c), where the 10.0% weight loss at about 200–400 °C was observed (the weight loss of about 8% at 550–700 °C resulted from the phase of Fe3O4 to Fe2O3). The presence of the abundant polyacrylate made the aqueous solution of PA–Fe3O4 nanoparticles very stable, and have a high negatively charged surface with a zeta potential of −43.0 mV (at the concentration of 1 g L−1). The PA–Fe3O4 nanoparticles displayed a saturation magnetization of 51.2 emu g−1, and superparamagnetism (the coercivity (14.81 Oe) and remanence (1.94 emu g−1) at 300 K were both negligible in the magnetic hysteresis loops of PA–Fe3O4, Fig. 1d). The TEM image (Fig. 1e) showed spherical PA–Fe3O4 nanoparticles with an average size of about 193 ± 20 nm, which was slightly smaller than the result from DLS measurements (205 nm, PDI 0.167, Fig. S1 in ESI†). Due to the relatively narrowly-distributed size, suitable polyacrylate coating and magnetic properties of PA–Fe3O4 nanoparticles, the aqueous solution of PA–Fe3O4 nanoparticles can
3370 | Analyst, 2015, 140, 3368–3374
be fast assembled to PCs with the assistance of a magnet. As shown in Fig. 1f, the same aqueous solution of PA–Fe3O4 nanoparticles can be assembled into PA–Fe3O4-MPCs with different brilliant colors under different magnetic field strengths (here the varied magnetic field strength was achieved by changing the distance between the solution of PA–Fe3O4 nanoparticles and the NdFeB magnet). The magnetic force FM acting on a magnetic nanoparticle in a magnetic field was calculated according to the following equation:42 F M ¼ μ0 V p M p rH
ð3Þ
where Vp is the volume of the particle, Mp is the magnetization in a given gradient magnetic field (∇H) and μ0 is the permeability of free space. According to Coulomb’s law, the electrostatic repulsive force FE between the charged nanoparticles is: F E ¼ kQq=d 2
ð4Þ
where k is the electrostatic force constant under the experimental conditions, Q and q are the quantity of pure electric charge of the interactional particles, and d is the distance between the interactional particles. For each PA–Fe3O4 nanoparticle stably-suspended in a magnetic field, FM and FE were balanced. In the same medium (e.g. the aqueous solution), the Vp was constant, so the longer distance from the NdFeB magnet to PA–Fe3O4 nanoparticle solution leads to the weaker magnetization Mp as well as weaker FM. For balance, the weaker FE, accordingly, the larger d value was expected as Q
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Analyst
and q kept constant in the same medium. Then, according to eqn (2), longer reflection wavelength λ was rational (d in eqn (4) is proportional to D in eqn (2)). Consequently, the red, orange, yellow, green, cyan, blue and purple colors were sequentially observed as the distance between the solution of PA–Fe3O4 nanoparticles and the NdFeB magnet was gradually changed from 5 cm to 1.5 cm. To further quantitatively describe the PA–Fe3O4-MPCs, the reflection spectra were measured (Fig. 2). As shown in Fig. 2a, the aqueous solution of PA–Fe3O4 nanoparticles at different concentrations can be magnetically assembled into PCs in the fixed magnetic field strength, but with different reflection wavelengths. The higher concentration of the PA–Fe3O4 nanoparticles made the PA–Fe3O4 nanoparticles closer to each other, which meant a shorter distance between PA–Fe3O4 nanoparticles (d in eqn (4)) and smaller distance of diffracting plane spacing of PCs (D in eqn (2)), accordingly, a shorter reflection wavelength of PA–Fe3O4-MPCs was observed (eqn (2)). For the fixed concentration of PA–Fe3O4 nanoparticles (e.g., 4 g L−1), the reflection wavelength was shorter as the NdFeB
Fig. 2 Reflection spectra corresponding to (a) different concentrations of PA–Fe3O4 nanoparticles at the fixed distance (3.0 cm) between NdFeB magnet and solution of PA–Fe3O4 nanoparticles. The peaks blueshift as the concentration increases from 2.0 to 18.0 g L−1 with step concentration of 2.0 g L−1 and (b) different external magnetic field strength achieved by changing the distance between NdFeB magnet and solution of PA–Fe3O4 nanoparticles (4.0 g L−1). The peaks blueshift as the distance decreases from 5.0 to 1.5 cm with step size of 0.5 cm.
This journal is © The Royal Society of Chemistry 2015
Paper
magnet was close to the solution of PA–Fe3O4 nanoparticles (Fig. 2b), which is in accordance with the photos in Fig. 1f. To obtain the good shape and intensity of PA–Fe3O4-MPCs, the concentration of PA–Fe3O4 nanoparticles was fixed at 4 g L−1 and the distance between the solution of PA–Fe3O4 nanoparticles and the NdFeB magnet was fixed at 3.0 cm for the subsequent measurements. 3.2.
IS assay of NaCl solution by PA–Fe3O4 MPCs
To evaluate the feasibility of PA–Fe3O4 MPCs for the IS assay, the most common electrolyte, NaCl, was used as a model analyte. First, the interaction dynamics of NaCl and PA–Fe3O4 MPCs were examined. As shown in Fig. 3, the reflection wavelength of PA–Fe3O4 MPCs was obviously blue-shifted upon the addition of NaCl solution within 5 s and reached the maximum blueshifts at 8 s. After that period, the wavelengths of the reflection spectra remained unchanged, but the intensity decreased gradually till 62 s (the lines of 62, 68, 74 and 80 s in Fig. 3 were overlayed). The reason behind the gradual decrease of the peak intensity is still unknown, but in this work and other research on PCs, only the reflection wavelength was concerned, so the varieties in intensity did not affect the sensing of IS. The reflection wavelengths of PA–Fe3O4 MPCs displayed continuous blueshifts against the IS of NaCl solution (Fig. 4). The change of the reflection wavelength Δλ upon the addition of NaCl solution (Δλ = λ − λ0, λ and λ0 were the wavelength in the presence and absence of NaCl respectively) was corrected with the IS of NaCl (INaCl, in the range of 0.05–0.35 mmol L−1) according to the equation Δλ = −4.43 − 407.1INaCl + 428.7INaCl2, where INaCl was in mmol L−1 (R2 = 0.9983, Fig. 4, insert). The averaged sensitivity in the range of 0.05–0.35 mmol L−1 was 266 nm L mmol−1. The relative standard deviation (RSD) of the Δλ at INaCl 0.10 mmol L−1 was 4.4% (n = 5). It was noteworthy that the wavelength blueshifts were more sensitive to INaCl in the low IS range (Fig. 4), for instance, the
Fig. 3 The time-dependent reflection spectra of PA–Fe3O4 MPCs upon the addition of NaCl solution (20 µL, 10 mmol L−1). The line of “0 s” referred to the blank PA–Fe3O4 MPCs.
Analyst, 2015, 140, 3368–3374 | 3371
View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Paper
Analyst
Fig. 4 Reflection spectra of PA–Fe3O4 MPCs in the absence and presence of NaCl at various IS. The concentration of PA–Fe3O4 nanoparticles was fixed at 4.0 g L−1 and the distance between the NdFeB magnet and PA–Fe3O4 nanoparticles solution was fixed at 3 cm. The lines from right to left were corresponding to the IS (NaCl) from 0 to 0.40 mmol L−1 with step concentration of 0.05 mmol L−1. Insert shows the relationship between Δλ and IS of NaCl solution.
sensitivity was about 453 nm L mmol−1 when INaCl was 0.05 mmol L−1. However, the wavelength blueshifts gradually became less sensitive to INaCl as INaCl increased to a higher level (the average sensitivity was about 294 nm L mmol−1 in 0.05–0.30 mmol L−1, Fig. 4). We first excluded the influence of gradual addition of NaCl solution, as the Δλ was identical to the final INaCl, regardless of whether the NaCl solution was gradually added or added once. The decreased sensitivity of Δλ against INaCl at higher INaCl was most probably related to the adsorption of Na+ onto the surface of PA–Fe3O4 nanoparticles. When INaCl is lower, each Na+ might have a better chance of attaching to the negatively charged surface of PA–Fe3O4 nanoparticles, and leading to a more effective decrease of the negative charges on the surface of PA–Fe3O4 nanoparticles, and thereby to a larger reduction in the distance of the nanoparticles, and finally to a higher sensitivity of Δλ against INaCl. The effect of temperature on sensing of IS by MPCs was evaluated (Fig. S2 in ESI†). The original reflection wavelength of MPCs was slightly blue-shifted when temperature was increased from 20 to 40 °C (top, Fig. S2 in ESI†), however, the MPCs at other temperatures also displayed wavelength blueshifts toward the increase of IS. Fig. S2† (below) shows the reflection wavelength change Δλ upon INaCl at 30 °C, where the sensitivity was further enhanced and the detectable IS range was slightly narrowed compared with that at 25 °C. 3.3.
IS assay of other electrolytes
To further examine the feasibility of the PA–Fe3O4 MPCs for the universal IS assay, other common electrolytes corresponding to the salinity of seawater (KCl, Na2SO4, MgCl2, CaCl2), HCl and NaOH and MgSO4 were involved as the IS contributor. As shown in Fig. 5, the presence of other electrolytes also led to the blueshifts of the reflection wavelength of PA–Fe3O4 MPCs. Besides, nearly all the electrolytes, except
3372 | Analyst, 2015, 140, 3368–3374
Fig. 5 The plot of electrolytes-induced Δλ of PA–Fe3O4 MPCs against the IS of different electrolytes and mixture of some electrolytes at different proportions (molar ratio). All data were measured at 4 g L−1 of PA–Fe3O4 nanoparticles and the 3.0 cm distance from the NdFeB magnet and the solution of PA–Fe3O4 nanoparticles.
NaOH, displayed very similar trends related to Δλ and the IS of solutions. The presence of NaOH, however, led to smaller blueshifts compared with other electrolytes, which was ascribed to the special properties of NaOH. On the one hand, the electrostatic adsorption of Na+ onto the exterior COO− of PA–Fe3O4 nanoparticles led to the decreased negative charges of PA– Fe3O4 nanoparticles; on the other hand, the OH− stimulated the dissociation of polyacrylate into COO−, leading to the increase of negative charges. Overall, the decrease of the negative charges in the presence of NaOH was not as much as the presence of NaCl, which was further proved by the Zeta potential measurements (Fig. S3 in ESI†). Consequently, the presence of NaOH contributed the little decrease of the distance (d in eqn (4)) between the Fe3O4 nanoparticles, as well as the smaller wavelength blueshifts of PA–Fe3O4 MPCs, as d in eqn (4) is proportional to D in eqn (2). To demonstrate whether the proposed MPCs work well in basic solution, we evaluated the tolerance of NaOH in NaCl solution for IS sensing. As shown in Fig. S4 (in ESI†), the NaOH–NaCl (1 : 4, 10 mM, pH = 11.3) solution had nearly the same response to the pure NaCl, thus the MPCs could measure the IS of NaCl solution at some basic pHs < 11.3 (with the final solution of MPCs pH 6.2). Note that the IS in all the figures herein were the final IS value in the measured MPCs solution. If we add the same volume of the NaCl solutions with different INaCl into the MPCs solution, we can plot the wavelength blueshifts to INaCl of the added solutions. In this way, the MPCs could work well for some solutions at basic pHs, as only a small volume (e.g. 5 μL) of the solution with higher IS was added into 1 mL of MPCs solution, which would lead to limited changes of the pH of the final solution. Besides, NaOH was not the contributor of IS and seawater salinity due to its high reactivity with carbon dioxide, thus the PA–Fe3O4 MPCs were of practical use for IS sensing. HCl (or acidity) was regarded as the common contributor to IS, so pH was not discussed as the single factor herein. Surprisingly, the wavelength blue-shifts of MPCs to HCl (for the final solution of HCl 0.05–0.30 mM corresponding to pH 6.03
This journal is © The Royal Society of Chemistry 2015
View Article Online
Analyst
Paper Table 1
Determination results of salinity in bohai seawater samples
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
ts Salinity (%, mean + pffiffiffi, n = 3, t = 4.30 for n 95% confidence level) Samples
This methoda
Electric conductivity
Chlorinity titrationb
Recoveryc (%)
S1 S2 S3 S4
2.69 ± 0.05 2.81 ± 0.10 1.87 ± 0.07 1.46 ± 0.07
2.63 ± 0.03 2.71 ± 0.03 1.90 ± 0.03 1.51 ± 0.03
2.67 ± 0.03 2.77 ± 0.07 1.99 ± 0.07 1.54 ± 0.03
104 104 94 92
Calculated from the IS in mol L−1 multiply the molar mass of NaCl. Calculated according to S‰ = 0.03 + 1.805Cl‰ c Recoveries for spiked 0.05 mmol L−1 of NaCl.
a b
Fig. 6 The fitting results of Δλ and IS of different electrolytes using PA– Fe3O4 MPCs as the sensing platform: (a) univalent, (b) divalent cations and (c) the fitting parameters in the equation of Δλ = C + B1Ix + B2Ix2, where Ix was the IS of the electrolytes solution in mmol L−1. All data were measured at 4 g L−1 of PA–Fe3O4 nanoparticles and the 3.0 cm distance from the NdFeB magnet and the solution of PA–Fe3O4 nanoparticles.
to 3.87) and NaCl were almost the same, which was in accordance with the results that addition of HCl and NaCl into the solution of PA–Fe3O4 nanoparticles has a similar effect on the Zeta potential (Fig. S3 in ESI†). The great difference between our results and previous publication31 was most probably ascribed to different PCs materials and the different concentration range of HCl. For a more accurate description of the responses of PA– Fe3O4 MPCs to the IS of different electrolytes, the fitting results of Δλ against IS are shown (Fig. 6). All data could be fitting according to the equation of Δλ = C + B1Ix + B2Ix2, where Ix is the IS of the electrolytes solution in mmol L−1. In Fig. 6a and b the fitting lines of univalent and divalent cations of electrolytes, respectively, are grouped, while Fig. 6c plots the fitting parameters (C, B1 and B2) related to the fitting lines of different electrolytes or the mixture of electrolytes at different proportions. All the fitting lines were nearly overlaid, and the variation of the fitting parameters was in a relative narrow range. All these data suggested that the PA– Fe3O4 MPCs had great potential for monitoring the IS of solutions, as the PA–Fe3O4 MPCs had the identical responses toward the common electrolytes corresponding to the salinity of seawater. 3.4.
Real sample IS assay
The developed PA–Fe3O4-MPCs based reflectometry method was applied for the determination of the IS or salinity of the Bohai Sea water samples (Table 1). For verification, the salinity of these seawater samples was also determined by the standard electric conductimetry method and chlorinity titration
This journal is © The Royal Society of Chemistry 2015
(Table 1). The salinity of these sea samples determined by our PA–Fe3O4-MPCs based reflectometry method was in good agreement with the other two standard methods, and the recoveries varied in the range of 92–104%, demonstrating the good accuracy of the developed method. The salinity of the seawater samples varied from 1.5% to 2.8%, corresponding to the seawater collected in the region from near to far from the beach. The low salinity of the seawater collected in the near the beach region might be ascribed to the adsorption of some ions by the beach-sands.
4.
Conclusion
We have developed a novel method for the rapid and sensitive IS assay based on the electrolyte-induced sensitive wavelength blueshifts of the reflection spectra of PA–Fe3O4-MPCs. The PA–Fe3O4-MPCs displayed wavelength blueshifts identical to the total IS of the common electrolytes corresponding to the salinity of seawater. Besides, the PA–Fe3O4-MPCs gave relatively high sensitivity at about 294 nm L mmol−1 in the range of 0.05–0.30 mmol L−1 and fast response (at 8 s) to the IS of aqueous solutions. To the best of our knowledge, this is the first work on PCs demonstrating an identical response to the total IS of the above-mentioned various salts (for the comparison with other PCs methods, please see Table S1 in ESI†). Consequently, the proposed PA–Fe3O4-MPCs would have great potential for the IS assay, and the PA–Fe3O4-MPCs would find application in the fundamental research on charge-related interactions. The adjustment of the PA–Fe3O4 particle size so as to obtain the visible-region wavelength would be more convenient for the real application of this kind of MPCs.
Acknowledgements This work was supported by the National Basic Research Program of China (no. 2011CB707703), the National Natural Science Foundation of China (no. 21175073) and the Tianjin Natural Science Foundation (no. 13JCYBJC17000).
Analyst, 2015, 140, 3368–3374 | 3373
View Article Online
Paper
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Notes and references 1 IUPAC. Compendium of Chemical Terminology, 2nd edn (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford, 1997. 2 J. R. Guzman-Sepulveda, V. I. Ruiz-Perez, M. Torres Cisneros, J. J. Sanchez-Mondragon and D. A. May-Arrioja, IEEE Photonics Technol. Lett., 2013, 25, 2323–2326. 3 Y. Zhao and Y. B. Liao, Sens. Actuators, B, 2002, 86, 63–67. 4 C. Wu, B. O. Guan, C. Lu and H. Y. Tam, Opt. Express, 2011, 19, 20003–20008. 5 H. A. Rahman, S. W. Harun, M. Yasin and H. Ahmad, IEEE J. Sel. Top. Quantum Electron., 2012, 18, 1529–1533. 6 L. V. Nguyen, M. Vasiliev and K. Alameh, IEEE Photonics Technol. Lett., 2011, 23, 450–452. 7 S. Wang, J. Wang, G. Li and L. Tong, Appl. Opt., 2012, 51, 3017–3023. 8 C. Fenzl, S. Wilhelm, T. Hirsch and O. S. Wolfbeis, ACS Appl. Mater. Interfaces, 2013, 5, 173–178. 9 E. Yablonovitch, Phys. Rev. Lett., 1987, 58, 2059–2062. 10 S. John, Phys. Rev. Lett., 1987, 58, 2486–2489. 11 Y. Zhao, Z. Xie, H. Gu, C. Zhu and Z. Gu, Chem. Soc. Rev., 2012, 41, 3297–3317. 12 S. Kinoshita and S. Yoshioka, ChemPhysChem, 2005, 6, 1442–1459. 13 Y. Takeoka, J. Mater. Chem., 2012, 22, 23299–23309. 14 C. Fenzl, T. Hirsch and O. S. Wolfbeis, Angew. Chem., Int. Ed., 2014, 53, 3318–3335. 15 I. B. Burgess, M. Loncar and J. Aizenberg, J. Mater. Chem. C, 2013, 1, 6075–6086. 16 Y. Takeoka, J. Mater. Chem. C, 2013, 1, 6059–6074. 17 W. Hong, X. Hu, B. Zhao, F. Zhang and D. Zhang, Adv. Mater., 2010, 22, 5043–5047. 18 B. Ye, F. Rong, H. Gu, Z. Xie, Y. Cheng, Y. Zhao and Z. Gu, Chem. Commun., 2013, 49, 5331–5333. 19 B.-F. Ye, Y.-J. Zhao, Y. Cheng, T.-T. Li, Z.-Y. Xie, X.-W. Zhao and Z.-Z. Gu, Nanoscale, 2012, 4, 5998–6003. 20 Y.-X. Zhang, P.-Y. Zhao and L.-P. Yu, Sens. Actuators, B, 2013, 181, 850–857. 21 X. Hu, Q. An, G. Li, S. Tao and J. Liu, Angew. Chem., Int. Ed., 2006, 45, 8145–8148.
3374 | Analyst, 2015, 140, 3368–3374
Analyst
22 R. Zhang, Y. Wang and L.-P. Yu, J. Hazard. Mater., 2014, 280, 46–54. 23 J. Cui, W. Zhu, N. Gao, J. Li, H. Yang, Y. Jiang, P. Seidel, B. J. Ravoo and G. Li, Angew. Chem., Int. Ed., 2014, 53, 3844–3848. 24 M. Liu and L. P. Yu, Analyst, 2013, 138, 3376–3379. 25 M. C. Chiappelli and R. C. Hayward, Adv. Mater., 2012, 24, 6100–6104. 26 M. Chen, L. Zhou, Y. Guan and Y. Zhang, Angew. Chem., Int. Ed., 2013, 52, 9961–9965. 27 Y. Dou, J. Han, T. Wang, M. Wei, D. G. Evans and X. Duan, J. Mater. Chem., 2012, 22, 14001–14007. 28 H. Hu, Q.-W. Chen, K. Cheng and J. Tang, J. Mater. Chem., 2012, 22, 1021–1027. 29 S.-N. Yin, C.-F. Wang, S.-S. Liu and S. Chen, J. Mater. Chem. C, 2013, 1, 4685–4690. 30 J. Shin, S. G. Han and W. Lee, Anal. Chim. Acta, 2012, 752, 87–93. 31 K. Lee and S. A. Asher, J. Am. Chem. Soc., 2000, 122, 9534– 9537. 32 J. Xu and Z. Guo, J. Colloid Interface Sci., 2013, 406, 1–17. 33 J. Y. Wang and J. T. Zhu, Eur. Polym. J., 2013, 49, 3420– 3433. 34 L. He, M. Wang, J. Ge and Y. Yin, Acc. Chem. Res., 2012, 45, 1431–1440. 35 H. S. Lee, J. H. Kim, J.-S. Lee, J. Y. Sim, J. Y. Seo, Y.-K. Oh, S.-M. Yang and S.-H. Kim, Adv. Mater., 2014, 26, 5734–5734. 36 J. Ge and Y. Yin, J. Mater. Chem., 2008, 18, 5041–5045. 37 W. Luo, H. Ma, F. Mou, M. Zhu, J. Yan and J. Guan, Adv. Mater., 2013, 26, 1058–1064. 38 J. P. Ge, Y. X. Hu and Y. D. Yin, Angew. Chem., Int. Ed., 2007, 46, 7428–7431. 39 C. Huber, I. Klimant, C. Krause, T. Werner, T. Mayr and O. S. Wolfbeis, Fresenius J. Anal. Chem., 2000, 368, 196–202. 40 S. H. Xuan, Y. X. J. Wang, J. C. Yu and K. C. F. Leung, Chem. Mater., 2009, 21, 5079–5087. 41 Y. Sun and H.-F. Wang, J. Chromatogr., A, 2013, 1316, 62–68. 42 C. Kotsmar, K. Y. Yoon, H. Yu, S. Y. Ryoo, J. Barth, S. Shao, M. a. Prodanović, T. E. Milner, S. L. Bryant, C. Huh and K. P. Johnston, Ind. Eng. Chem. Res., 2010, 49, 12435–12443.
This journal is © The Royal Society of Chemistry 2015
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
PAPER
View Journal | View Issue
Ionic strength assay via polyacrylate–ferriferrous oxide magnetic photonic crystals†
Cite this: Analyst, 2015, 140, 3368
Yan-Ran Li, Ye Sun and He-Fang Wang* Convenient reading out and/or determination of ionic strength (IS) is of great significance for both scientific research and real life applications. We presented here a novel method for the rapid and sensitive IS assay based on the electrolyte-induced sensitive wavelength blueshifts of the reflection spectra of polyacrylate capped Fe3O4 magnetic photonic crystals (PA–Fe3O4-MPCs). For HCl, MgSO4 and the common electrolytes corresponding to the salinity of seawater (including NaCl, KCl, MgCl2, CaCl2, Na2SO4 and their mixtures), the PA–Fe3O4-MPCs displayed wavelength blueshifts identical to the total IS of the aqueous solutions, regardless of the kind of above-mentioned electrolytes in the solutions. Besides, the PA–Fe3O4-MPCs exhibited relatively high sensitivity (an average of 294 nm L mmol−1 in the range of 0.05–0.30 mmol L−1, and an even higher value of 386 nm L mmol−1 at 0.05–0.15 mmol L−1) and fast response (within 8 s) to the IS of aqueous solutions. The relative standard deviation (RSD) for IS (NaCl, 0.1 mmol L−1) was 4.4% (n = 5). The developed method was applied to determine the salinity of seawater
Received 2nd February 2015, Accepted 20th March 2015
samples, and the determined results were validated by the traditional standard chlorinity titration and
DOI: 10.1039/c5an00218d
electric conductimetry method. The recoveries were in the range of 92–104%. The proposed PA–Fe3O4MPCs based reflectometry method would have great potential for IS and salinity assays.
www.rsc.org/analyst
1.
Introduction
The ionic strength (IS) of a solution is a function of the concentration of all ions in that solution, calculated as: I¼
n 1X ci zi 2 2 i¼1
ð1Þ
where ci is the concentration of ion i (often in M, mol L−1) and zi is the charge number of that ion.1 IS plays an important role in Debye–Hückel theory and double-layer theory and relates to electrokinetic and electroacoustic phenomena in colloids and other heterogeneous systems.2 Besides, IS monitoring is important for natural aquatic ecosystems, climate models and chemical oceanography.2 Consequently, convenient reading out and/or determination of IS is of great significance for both scientific research and real life application. To date, several methods based on different principles have been used for the measurement of IS (or salinity), including traditional chlorinity titration, electric conductimetry,2 fiber Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin Key laboratory of Biosensing and Molecular Recognition (Nankai University), State Key Laboratory of Medicinal Chemical Biology (Nankai University), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 94 Weijin Road, Tianjin 300071, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5an00218d
3368 | Analyst, 2015, 140, 3368–3374
optical sensors,3 photonic crystal fibers,4 displacement/differential measurements,5 interferometry,6 fiber resonators7 and photonic crystal reflectometry.8 Every method has its pros and cons; photonic crystal reflectometry has the merits of nakedeye inspection and is thus promising for convenient monitoring and detection of IS (or salinity).8 However, only a few papers on photonic crystal reflectometry have been published, where poly(styrene-co-sodium styrenesulfonate) templated polyacrylamide hydrogel was used as the sensing material.8 Photonic crystals (PCs), consisting of periodical structures of different dielectric materials, were first reported by Yablonovitch9 and John.10 The most noteworthy feature of PCs is their colorful appearance,11–13 which arises from interference and reflection according to the Bragg’s and Snell’s laws:14 λ ¼ 2Dðneff 2 sin2 θÞ1=2
ð2Þ
where λ is the wavelength of the reflected light, D is the distance of diffracting plane spacing, neff is the average refractive index of the PCs, and θ is the angle of incident light. The dependence of λ on the features of PC materials (e.g., D, neff, and θ) enables the design and application of various PC sensors. Recently, PCs have been used for sensing various external stimuli15,16 such as pressure,15 coexistence substances,17–24 temperature,25,26 humidity,27–29 pH,30,31 and ionic strength.26,31 PCs used were mainly fabricated by nanolithograph and
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Analyst
colloidal assembly methods,32 therein colloidal assembly was usually assisted by gravity, electricity and magnetism.33 For real application of colloidal assembled PCs, the solidification of periodic structure was always required. However, the solidification process, usually involving the use of cross-linking monomers, initiators and other additives, might result in the disassembly of the periodic structure of PCs.26 Magnetic PCs, assembled via the superparamagnetic colloidal Fe3O4,34,35 have fast-assembly and solidification-free brilliant structural colors. Besides, the colloidal Fe3O4 can be easily prepared by the solvothermal method, and the resulting Fe3O4 can be homogeneously dispersed in some solvents and magnetically self-assembled to PCs, thanks to the balance of electrostatic repulsive force36/steric repulsion37 and magnetic force. The Fe3O4 PCs have been pioneered by Yin et al.36,38 but the real application of Fe3O4 PCs for optical sensing of IS has not been reported so far. Herein, we represented a novel method for the rapid and sensitive determination of IS based on the electrolytesinduced sensitive wavelength shifts of the reflection spectra of polyacrylate capped Fe3O4 magnetic PCs (PA–Fe3O4-MPCs). The HCl, NaOH, MgSO4 and the most common electrolytes corresponding to the salinity of seawater39 (including sodium chloride, potassium chloride, sodium sulfate, magnesium chloride, calcium chloride and their mixtures at different proportions), were involved in evaluating the feasibility of this IS sensor. Interestingly, the PA–Fe3O4-MPCs displayed wavelength-shifts almost identical to the total IS of the solutions composed of HCl, MgSO4 and/or the electrolytes corresponding to the salinity of seawater. Besides, the PA–Fe3O4MPCs sensor displayed rapid and sensitive response to the IS of aqueous solution. Consequently, the developed PA–Fe3O4MPCs sensor would have great potential for IS and salinity assays.
2. Experimental 2.1.
Reagents
All reagents (at least of analytical grade) are used directly. Iron(III) chloride hexahydrate (FeCl3·6H2O, 97%, Aladdin, Shanghai, China), anhydrous sodium acetate (99%, Aladdin, Shanghai, China), sodium acrylate (99%, Heowns, Tianjin, China) and ethylene glycol (EG, Aladdin, Shanghai, China) were used for preparing polyacrylate capped Fe3O4 (PA–Fe3O4) nanoparticles. The electrolytes including sodium chloride, potassium chloride, sodium sulfate, magnesium chloride, calcium chloride and magnesium sulfate were all purchased from Guangfu fine Chemical Research Institute (Tianjin, China). Silver nitrate (99%, Aladdin, Shanghai, China) was used for titration of chloride ions and potassium dichromate (Guangfu fine Chemical Research Institute, Tianjin, China) was used as the indicator for monitoring the titration end point. All solutions were newly prepared in ultrapure water (Wahaha, Hangzhou, China). The standard NdFeB magnet was used throughout all the experiments.
This journal is © The Royal Society of Chemistry 2015
Paper
2.2.
Apparatus
The morphology of PA–Fe3O4 nanoparticles was observed on a JEOL 100 CXII transmission electron microscope (TEM) operating at a 200 kV accelerating voltage. The samples for TEM were prepared by drying sample droplets from ultrapure water dispersion onto a 200-mesh Cu grid coated with a lacey carbon film (Zhongjingkeyi Technology, Beijing, China). The X-ray diffraction (XRD) spectra were recorded with a Rigaku D/max2500 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5418 Å). The Fourier transform infrared (FT-IR) spectra (4000–400 cm−1) in KBr were recorded with a Nicolet MAGNA-560 FTIR spectrometer (Nicolet, Madison, WI). The magnetic properties (M-H curve) were studied using a LDJ 9600-1 vibrating sample magnetometer (LDJ Electronics Inc., Troy, MI, USA) at room temperature by cycling the field from −6 to 6 kOe. The thermogravimetric analysis (TGA) was conducted with a PTC-10A TG-DTA thermo-analyzer (Rigaku, Japan). The Zeta potentials and dynamic light scattering (DLS) were measured on a laser light scattering spectrometer Zetasizer Nano ZS (Malvern, UK) at 25 °C. The reflection spectra of the PCs were measured on an HR4000CG-UV-VIS-NIR spectrometer (Ocean Optics, FL, US). The DDSJ-308A conductivity meter (YiChen Technologies, Xiamen, China) was used for determination of the salinity of seawater samples. 2.3.
Synthesis of PA–Fe3O4 nanoparticles
The PA–Fe3O4 nanoparticles were synthesized according to the literature.40,41 In a 100 mL flask, FeCl3·6H2O (0.54 g) and EG (20 mL) were mixed by magnetic stirring to obtain a homogeneous yellow solution, then sodium acrylate (0.3 g) and CH3COONa (1.2 g) were added successively and kept stirring for 1.5 h. The resulting viscous yellow solution was transferred into a Teflon-lined stainless-steel autoclave and heated at 200 °C for 10 h, and then the autoclave was cooled to room temperature. Finally, the black products were washed with 40 mL of pure water–ethanol (1/1, V/V), 30 mL of pure water and 30 mL of ethanol respectively, and the resulting solid was dried under vacuum overnight. 2.4.
Reflection measurements
All measurements were carried out on an HR4000CG-UV-VIS-NIR spectrometer equipped with a reflection probe. Typically, the reflection probe was placed perpendicular to the glass vessel containing the aqueous PA–Fe3O4 nanoparticles solution (1 mL, the solution was subject to ultrasound for 5 min before use), and the NdFeB magnet was placed in the direction opposite to the reflection probe (in the other side of the glass vessel). For optimization of the measurement conditions, the NdFeB magnet was placed at different distances (5.0–1.5 cm) from the 1 mL solution of PA–Fe3O4 nanoparticles (with various concentrations of 2.0–18.0 g L−1). For the IS assay, the concentration of PA–Fe3O4 nanoparticles solution was fixed at 4.0 g L−1 and the distance between the NdFeB magnet and PA–Fe3O4 nanoparticles solution was fixed at 3 cm. Different electrolyte solutions (IS 10 mmol L−1, 5 μL each time) were
Analyst, 2015, 140, 3368–3374 | 3369
View Article Online
Paper
Analyst
added into the PA–Fe3O4-MPCs solution (1 mL, 4.0 g L−1) and stirred gently by the pipette tip for 2 s, and then the corresponding reflection spectrum was recorded 6 s later. The reflection measurements were done at 25 °C, or as otherwise stated.
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
2.5.
Real samples analysis
The samples were collected from different regions of the Bohai Sea (Tianjin, China). All samples were filtered through 0.45 μm Supor filter and stored in the precleaned glass bottles at 4 °C fridge. As the average salinity in the world’s ocean is as high as 3.5% (35 g L−1, or 599 mM), all the samples were diluted 20 times before determination. Typically, 2 μL of the diluted seawater sample was added into 1 mL of PA–Fe3O4 MPCs (4.0 g L−1) solution and the reflection spectra were measured by the above-mentioned procedure. For validation of the proposed method, 0.5 mol L−1 of NaCl was spiked in the filtered seawater samples, and 2 μL of the 20-time diluted spike-samples were added into 1 mL of PA–Fe3O4 MPCs (4.0 g L−1) solution to determine the recoveries. For verification, the salinity of the seawater samples was also determined by the standard electric conductimetry method and chlorinity titration, where the filtered seawater samples were 20-time diluted before determination.
Fig. 1 (a) XRD pattern, (b) FTIR spectrum, (c) TGA curve, (d) magnetic hysteresis curve, and (e) TEM of the PA–Fe3O4 nanoparticles, and (f ) digital photos of the PA–Fe3O4-MPCs with 4 g L−1 of PA–Fe3O4 nanoparticles assembled under different magnet intensities (by changing the distance between PA–Fe3O4 nanoparticles solution and NdFeB magnet, 5 to 1.5 cm, left to right).
3. Results and discussion 3.1. Characterization of PA–Fe3O4 nanoparticles and PA–Fe3O4-MPCs The prepared PA–Fe3O4 nanoparticles displayed typical face center cubic phase of Fe3O4 (JCPDS no. 19-0629) as shown in the XRD pattern (Fig. 1a). The strong peak at 580 cm−1 in the FTIR spectrum (Fig. 1b) was the hallmark of Fe–O stretching vibration, also demonstrating the existence of Fe3O4. The peaks around 1559 and 1410 cm−1 (Fig. 1b) corresponding to the COO− asymmetrical and symmetric stretching vibration indicated the presence of a polyacrylate coating. The existence of a polyacrylate coating was further ascertained by the TGA curve (Fig. 1c), where the 10.0% weight loss at about 200–400 °C was observed (the weight loss of about 8% at 550–700 °C resulted from the phase of Fe3O4 to Fe2O3). The presence of the abundant polyacrylate made the aqueous solution of PA–Fe3O4 nanoparticles very stable, and have a high negatively charged surface with a zeta potential of −43.0 mV (at the concentration of 1 g L−1). The PA–Fe3O4 nanoparticles displayed a saturation magnetization of 51.2 emu g−1, and superparamagnetism (the coercivity (14.81 Oe) and remanence (1.94 emu g−1) at 300 K were both negligible in the magnetic hysteresis loops of PA–Fe3O4, Fig. 1d). The TEM image (Fig. 1e) showed spherical PA–Fe3O4 nanoparticles with an average size of about 193 ± 20 nm, which was slightly smaller than the result from DLS measurements (205 nm, PDI 0.167, Fig. S1 in ESI†). Due to the relatively narrowly-distributed size, suitable polyacrylate coating and magnetic properties of PA–Fe3O4 nanoparticles, the aqueous solution of PA–Fe3O4 nanoparticles can
3370 | Analyst, 2015, 140, 3368–3374
be fast assembled to PCs with the assistance of a magnet. As shown in Fig. 1f, the same aqueous solution of PA–Fe3O4 nanoparticles can be assembled into PA–Fe3O4-MPCs with different brilliant colors under different magnetic field strengths (here the varied magnetic field strength was achieved by changing the distance between the solution of PA–Fe3O4 nanoparticles and the NdFeB magnet). The magnetic force FM acting on a magnetic nanoparticle in a magnetic field was calculated according to the following equation:42 F M ¼ μ0 V p M p rH
ð3Þ
where Vp is the volume of the particle, Mp is the magnetization in a given gradient magnetic field (∇H) and μ0 is the permeability of free space. According to Coulomb’s law, the electrostatic repulsive force FE between the charged nanoparticles is: F E ¼ kQq=d 2
ð4Þ
where k is the electrostatic force constant under the experimental conditions, Q and q are the quantity of pure electric charge of the interactional particles, and d is the distance between the interactional particles. For each PA–Fe3O4 nanoparticle stably-suspended in a magnetic field, FM and FE were balanced. In the same medium (e.g. the aqueous solution), the Vp was constant, so the longer distance from the NdFeB magnet to PA–Fe3O4 nanoparticle solution leads to the weaker magnetization Mp as well as weaker FM. For balance, the weaker FE, accordingly, the larger d value was expected as Q
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Analyst
and q kept constant in the same medium. Then, according to eqn (2), longer reflection wavelength λ was rational (d in eqn (4) is proportional to D in eqn (2)). Consequently, the red, orange, yellow, green, cyan, blue and purple colors were sequentially observed as the distance between the solution of PA–Fe3O4 nanoparticles and the NdFeB magnet was gradually changed from 5 cm to 1.5 cm. To further quantitatively describe the PA–Fe3O4-MPCs, the reflection spectra were measured (Fig. 2). As shown in Fig. 2a, the aqueous solution of PA–Fe3O4 nanoparticles at different concentrations can be magnetically assembled into PCs in the fixed magnetic field strength, but with different reflection wavelengths. The higher concentration of the PA–Fe3O4 nanoparticles made the PA–Fe3O4 nanoparticles closer to each other, which meant a shorter distance between PA–Fe3O4 nanoparticles (d in eqn (4)) and smaller distance of diffracting plane spacing of PCs (D in eqn (2)), accordingly, a shorter reflection wavelength of PA–Fe3O4-MPCs was observed (eqn (2)). For the fixed concentration of PA–Fe3O4 nanoparticles (e.g., 4 g L−1), the reflection wavelength was shorter as the NdFeB
Fig. 2 Reflection spectra corresponding to (a) different concentrations of PA–Fe3O4 nanoparticles at the fixed distance (3.0 cm) between NdFeB magnet and solution of PA–Fe3O4 nanoparticles. The peaks blueshift as the concentration increases from 2.0 to 18.0 g L−1 with step concentration of 2.0 g L−1 and (b) different external magnetic field strength achieved by changing the distance between NdFeB magnet and solution of PA–Fe3O4 nanoparticles (4.0 g L−1). The peaks blueshift as the distance decreases from 5.0 to 1.5 cm with step size of 0.5 cm.
This journal is © The Royal Society of Chemistry 2015
Paper
magnet was close to the solution of PA–Fe3O4 nanoparticles (Fig. 2b), which is in accordance with the photos in Fig. 1f. To obtain the good shape and intensity of PA–Fe3O4-MPCs, the concentration of PA–Fe3O4 nanoparticles was fixed at 4 g L−1 and the distance between the solution of PA–Fe3O4 nanoparticles and the NdFeB magnet was fixed at 3.0 cm for the subsequent measurements. 3.2.
IS assay of NaCl solution by PA–Fe3O4 MPCs
To evaluate the feasibility of PA–Fe3O4 MPCs for the IS assay, the most common electrolyte, NaCl, was used as a model analyte. First, the interaction dynamics of NaCl and PA–Fe3O4 MPCs were examined. As shown in Fig. 3, the reflection wavelength of PA–Fe3O4 MPCs was obviously blue-shifted upon the addition of NaCl solution within 5 s and reached the maximum blueshifts at 8 s. After that period, the wavelengths of the reflection spectra remained unchanged, but the intensity decreased gradually till 62 s (the lines of 62, 68, 74 and 80 s in Fig. 3 were overlayed). The reason behind the gradual decrease of the peak intensity is still unknown, but in this work and other research on PCs, only the reflection wavelength was concerned, so the varieties in intensity did not affect the sensing of IS. The reflection wavelengths of PA–Fe3O4 MPCs displayed continuous blueshifts against the IS of NaCl solution (Fig. 4). The change of the reflection wavelength Δλ upon the addition of NaCl solution (Δλ = λ − λ0, λ and λ0 were the wavelength in the presence and absence of NaCl respectively) was corrected with the IS of NaCl (INaCl, in the range of 0.05–0.35 mmol L−1) according to the equation Δλ = −4.43 − 407.1INaCl + 428.7INaCl2, where INaCl was in mmol L−1 (R2 = 0.9983, Fig. 4, insert). The averaged sensitivity in the range of 0.05–0.35 mmol L−1 was 266 nm L mmol−1. The relative standard deviation (RSD) of the Δλ at INaCl 0.10 mmol L−1 was 4.4% (n = 5). It was noteworthy that the wavelength blueshifts were more sensitive to INaCl in the low IS range (Fig. 4), for instance, the
Fig. 3 The time-dependent reflection spectra of PA–Fe3O4 MPCs upon the addition of NaCl solution (20 µL, 10 mmol L−1). The line of “0 s” referred to the blank PA–Fe3O4 MPCs.
Analyst, 2015, 140, 3368–3374 | 3371
View Article Online
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Paper
Analyst
Fig. 4 Reflection spectra of PA–Fe3O4 MPCs in the absence and presence of NaCl at various IS. The concentration of PA–Fe3O4 nanoparticles was fixed at 4.0 g L−1 and the distance between the NdFeB magnet and PA–Fe3O4 nanoparticles solution was fixed at 3 cm. The lines from right to left were corresponding to the IS (NaCl) from 0 to 0.40 mmol L−1 with step concentration of 0.05 mmol L−1. Insert shows the relationship between Δλ and IS of NaCl solution.
sensitivity was about 453 nm L mmol−1 when INaCl was 0.05 mmol L−1. However, the wavelength blueshifts gradually became less sensitive to INaCl as INaCl increased to a higher level (the average sensitivity was about 294 nm L mmol−1 in 0.05–0.30 mmol L−1, Fig. 4). We first excluded the influence of gradual addition of NaCl solution, as the Δλ was identical to the final INaCl, regardless of whether the NaCl solution was gradually added or added once. The decreased sensitivity of Δλ against INaCl at higher INaCl was most probably related to the adsorption of Na+ onto the surface of PA–Fe3O4 nanoparticles. When INaCl is lower, each Na+ might have a better chance of attaching to the negatively charged surface of PA–Fe3O4 nanoparticles, and leading to a more effective decrease of the negative charges on the surface of PA–Fe3O4 nanoparticles, and thereby to a larger reduction in the distance of the nanoparticles, and finally to a higher sensitivity of Δλ against INaCl. The effect of temperature on sensing of IS by MPCs was evaluated (Fig. S2 in ESI†). The original reflection wavelength of MPCs was slightly blue-shifted when temperature was increased from 20 to 40 °C (top, Fig. S2 in ESI†), however, the MPCs at other temperatures also displayed wavelength blueshifts toward the increase of IS. Fig. S2† (below) shows the reflection wavelength change Δλ upon INaCl at 30 °C, where the sensitivity was further enhanced and the detectable IS range was slightly narrowed compared with that at 25 °C. 3.3.
IS assay of other electrolytes
To further examine the feasibility of the PA–Fe3O4 MPCs for the universal IS assay, other common electrolytes corresponding to the salinity of seawater (KCl, Na2SO4, MgCl2, CaCl2), HCl and NaOH and MgSO4 were involved as the IS contributor. As shown in Fig. 5, the presence of other electrolytes also led to the blueshifts of the reflection wavelength of PA–Fe3O4 MPCs. Besides, nearly all the electrolytes, except
3372 | Analyst, 2015, 140, 3368–3374
Fig. 5 The plot of electrolytes-induced Δλ of PA–Fe3O4 MPCs against the IS of different electrolytes and mixture of some electrolytes at different proportions (molar ratio). All data were measured at 4 g L−1 of PA–Fe3O4 nanoparticles and the 3.0 cm distance from the NdFeB magnet and the solution of PA–Fe3O4 nanoparticles.
NaOH, displayed very similar trends related to Δλ and the IS of solutions. The presence of NaOH, however, led to smaller blueshifts compared with other electrolytes, which was ascribed to the special properties of NaOH. On the one hand, the electrostatic adsorption of Na+ onto the exterior COO− of PA–Fe3O4 nanoparticles led to the decreased negative charges of PA– Fe3O4 nanoparticles; on the other hand, the OH− stimulated the dissociation of polyacrylate into COO−, leading to the increase of negative charges. Overall, the decrease of the negative charges in the presence of NaOH was not as much as the presence of NaCl, which was further proved by the Zeta potential measurements (Fig. S3 in ESI†). Consequently, the presence of NaOH contributed the little decrease of the distance (d in eqn (4)) between the Fe3O4 nanoparticles, as well as the smaller wavelength blueshifts of PA–Fe3O4 MPCs, as d in eqn (4) is proportional to D in eqn (2). To demonstrate whether the proposed MPCs work well in basic solution, we evaluated the tolerance of NaOH in NaCl solution for IS sensing. As shown in Fig. S4 (in ESI†), the NaOH–NaCl (1 : 4, 10 mM, pH = 11.3) solution had nearly the same response to the pure NaCl, thus the MPCs could measure the IS of NaCl solution at some basic pHs < 11.3 (with the final solution of MPCs pH 6.2). Note that the IS in all the figures herein were the final IS value in the measured MPCs solution. If we add the same volume of the NaCl solutions with different INaCl into the MPCs solution, we can plot the wavelength blueshifts to INaCl of the added solutions. In this way, the MPCs could work well for some solutions at basic pHs, as only a small volume (e.g. 5 μL) of the solution with higher IS was added into 1 mL of MPCs solution, which would lead to limited changes of the pH of the final solution. Besides, NaOH was not the contributor of IS and seawater salinity due to its high reactivity with carbon dioxide, thus the PA–Fe3O4 MPCs were of practical use for IS sensing. HCl (or acidity) was regarded as the common contributor to IS, so pH was not discussed as the single factor herein. Surprisingly, the wavelength blue-shifts of MPCs to HCl (for the final solution of HCl 0.05–0.30 mM corresponding to pH 6.03
This journal is © The Royal Society of Chemistry 2015
View Article Online
Analyst
Paper Table 1
Determination results of salinity in bohai seawater samples
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
ts Salinity (%, mean + pffiffiffi, n = 3, t = 4.30 for n 95% confidence level) Samples
This methoda
Electric conductivity
Chlorinity titrationb
Recoveryc (%)
S1 S2 S3 S4
2.69 ± 0.05 2.81 ± 0.10 1.87 ± 0.07 1.46 ± 0.07
2.63 ± 0.03 2.71 ± 0.03 1.90 ± 0.03 1.51 ± 0.03
2.67 ± 0.03 2.77 ± 0.07 1.99 ± 0.07 1.54 ± 0.03
104 104 94 92
Calculated from the IS in mol L−1 multiply the molar mass of NaCl. Calculated according to S‰ = 0.03 + 1.805Cl‰ c Recoveries for spiked 0.05 mmol L−1 of NaCl.
a b
Fig. 6 The fitting results of Δλ and IS of different electrolytes using PA– Fe3O4 MPCs as the sensing platform: (a) univalent, (b) divalent cations and (c) the fitting parameters in the equation of Δλ = C + B1Ix + B2Ix2, where Ix was the IS of the electrolytes solution in mmol L−1. All data were measured at 4 g L−1 of PA–Fe3O4 nanoparticles and the 3.0 cm distance from the NdFeB magnet and the solution of PA–Fe3O4 nanoparticles.
to 3.87) and NaCl were almost the same, which was in accordance with the results that addition of HCl and NaCl into the solution of PA–Fe3O4 nanoparticles has a similar effect on the Zeta potential (Fig. S3 in ESI†). The great difference between our results and previous publication31 was most probably ascribed to different PCs materials and the different concentration range of HCl. For a more accurate description of the responses of PA– Fe3O4 MPCs to the IS of different electrolytes, the fitting results of Δλ against IS are shown (Fig. 6). All data could be fitting according to the equation of Δλ = C + B1Ix + B2Ix2, where Ix is the IS of the electrolytes solution in mmol L−1. In Fig. 6a and b the fitting lines of univalent and divalent cations of electrolytes, respectively, are grouped, while Fig. 6c plots the fitting parameters (C, B1 and B2) related to the fitting lines of different electrolytes or the mixture of electrolytes at different proportions. All the fitting lines were nearly overlaid, and the variation of the fitting parameters was in a relative narrow range. All these data suggested that the PA– Fe3O4 MPCs had great potential for monitoring the IS of solutions, as the PA–Fe3O4 MPCs had the identical responses toward the common electrolytes corresponding to the salinity of seawater. 3.4.
Real sample IS assay
The developed PA–Fe3O4-MPCs based reflectometry method was applied for the determination of the IS or salinity of the Bohai Sea water samples (Table 1). For verification, the salinity of these seawater samples was also determined by the standard electric conductimetry method and chlorinity titration
This journal is © The Royal Society of Chemistry 2015
(Table 1). The salinity of these sea samples determined by our PA–Fe3O4-MPCs based reflectometry method was in good agreement with the other two standard methods, and the recoveries varied in the range of 92–104%, demonstrating the good accuracy of the developed method. The salinity of the seawater samples varied from 1.5% to 2.8%, corresponding to the seawater collected in the region from near to far from the beach. The low salinity of the seawater collected in the near the beach region might be ascribed to the adsorption of some ions by the beach-sands.
4.
Conclusion
We have developed a novel method for the rapid and sensitive IS assay based on the electrolyte-induced sensitive wavelength blueshifts of the reflection spectra of PA–Fe3O4-MPCs. The PA–Fe3O4-MPCs displayed wavelength blueshifts identical to the total IS of the common electrolytes corresponding to the salinity of seawater. Besides, the PA–Fe3O4-MPCs gave relatively high sensitivity at about 294 nm L mmol−1 in the range of 0.05–0.30 mmol L−1 and fast response (at 8 s) to the IS of aqueous solutions. To the best of our knowledge, this is the first work on PCs demonstrating an identical response to the total IS of the above-mentioned various salts (for the comparison with other PCs methods, please see Table S1 in ESI†). Consequently, the proposed PA–Fe3O4-MPCs would have great potential for the IS assay, and the PA–Fe3O4-MPCs would find application in the fundamental research on charge-related interactions. The adjustment of the PA–Fe3O4 particle size so as to obtain the visible-region wavelength would be more convenient for the real application of this kind of MPCs.
Acknowledgements This work was supported by the National Basic Research Program of China (no. 2011CB707703), the National Natural Science Foundation of China (no. 21175073) and the Tianjin Natural Science Foundation (no. 13JCYBJC17000).
Analyst, 2015, 140, 3368–3374 | 3373
View Article Online
Paper
Published on 20 March 2015. Downloaded by Georgetown University Library on 28/05/2015 09:03:12.
Notes and references 1 IUPAC. Compendium of Chemical Terminology, 2nd edn (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford, 1997. 2 J. R. Guzman-Sepulveda, V. I. Ruiz-Perez, M. Torres Cisneros, J. J. Sanchez-Mondragon and D. A. May-Arrioja, IEEE Photonics Technol. Lett., 2013, 25, 2323–2326. 3 Y. Zhao and Y. B. Liao, Sens. Actuators, B, 2002, 86, 63–67. 4 C. Wu, B. O. Guan, C. Lu and H. Y. Tam, Opt. Express, 2011, 19, 20003–20008. 5 H. A. Rahman, S. W. Harun, M. Yasin and H. Ahmad, IEEE J. Sel. Top. Quantum Electron., 2012, 18, 1529–1533. 6 L. V. Nguyen, M. Vasiliev and K. Alameh, IEEE Photonics Technol. Lett., 2011, 23, 450–452. 7 S. Wang, J. Wang, G. Li and L. Tong, Appl. Opt., 2012, 51, 3017–3023. 8 C. Fenzl, S. Wilhelm, T. Hirsch and O. S. Wolfbeis, ACS Appl. Mater. Interfaces, 2013, 5, 173–178. 9 E. Yablonovitch, Phys. Rev. Lett., 1987, 58, 2059–2062. 10 S. John, Phys. Rev. Lett., 1987, 58, 2486–2489. 11 Y. Zhao, Z. Xie, H. Gu, C. Zhu and Z. Gu, Chem. Soc. Rev., 2012, 41, 3297–3317. 12 S. Kinoshita and S. Yoshioka, ChemPhysChem, 2005, 6, 1442–1459. 13 Y. Takeoka, J. Mater. Chem., 2012, 22, 23299–23309. 14 C. Fenzl, T. Hirsch and O. S. Wolfbeis, Angew. Chem., Int. Ed., 2014, 53, 3318–3335. 15 I. B. Burgess, M. Loncar and J. Aizenberg, J. Mater. Chem. C, 2013, 1, 6075–6086. 16 Y. Takeoka, J. Mater. Chem. C, 2013, 1, 6059–6074. 17 W. Hong, X. Hu, B. Zhao, F. Zhang and D. Zhang, Adv. Mater., 2010, 22, 5043–5047. 18 B. Ye, F. Rong, H. Gu, Z. Xie, Y. Cheng, Y. Zhao and Z. Gu, Chem. Commun., 2013, 49, 5331–5333. 19 B.-F. Ye, Y.-J. Zhao, Y. Cheng, T.-T. Li, Z.-Y. Xie, X.-W. Zhao and Z.-Z. Gu, Nanoscale, 2012, 4, 5998–6003. 20 Y.-X. Zhang, P.-Y. Zhao and L.-P. Yu, Sens. Actuators, B, 2013, 181, 850–857. 21 X. Hu, Q. An, G. Li, S. Tao and J. Liu, Angew. Chem., Int. Ed., 2006, 45, 8145–8148.
3374 | Analyst, 2015, 140, 3368–3374
Analyst
22 R. Zhang, Y. Wang and L.-P. Yu, J. Hazard. Mater., 2014, 280, 46–54. 23 J. Cui, W. Zhu, N. Gao, J. Li, H. Yang, Y. Jiang, P. Seidel, B. J. Ravoo and G. Li, Angew. Chem., Int. Ed., 2014, 53, 3844–3848. 24 M. Liu and L. P. Yu, Analyst, 2013, 138, 3376–3379. 25 M. C. Chiappelli and R. C. Hayward, Adv. Mater., 2012, 24, 6100–6104. 26 M. Chen, L. Zhou, Y. Guan and Y. Zhang, Angew. Chem., Int. Ed., 2013, 52, 9961–9965. 27 Y. Dou, J. Han, T. Wang, M. Wei, D. G. Evans and X. Duan, J. Mater. Chem., 2012, 22, 14001–14007. 28 H. Hu, Q.-W. Chen, K. Cheng and J. Tang, J. Mater. Chem., 2012, 22, 1021–1027. 29 S.-N. Yin, C.-F. Wang, S.-S. Liu and S. Chen, J. Mater. Chem. C, 2013, 1, 4685–4690. 30 J. Shin, S. G. Han and W. Lee, Anal. Chim. Acta, 2012, 752, 87–93. 31 K. Lee and S. A. Asher, J. Am. Chem. Soc., 2000, 122, 9534– 9537. 32 J. Xu and Z. Guo, J. Colloid Interface Sci., 2013, 406, 1–17. 33 J. Y. Wang and J. T. Zhu, Eur. Polym. J., 2013, 49, 3420– 3433. 34 L. He, M. Wang, J. Ge and Y. Yin, Acc. Chem. Res., 2012, 45, 1431–1440. 35 H. S. Lee, J. H. Kim, J.-S. Lee, J. Y. Sim, J. Y. Seo, Y.-K. Oh, S.-M. Yang and S.-H. Kim, Adv. Mater., 2014, 26, 5734–5734. 36 J. Ge and Y. Yin, J. Mater. Chem., 2008, 18, 5041–5045. 37 W. Luo, H. Ma, F. Mou, M. Zhu, J. Yan and J. Guan, Adv. Mater., 2013, 26, 1058–1064. 38 J. P. Ge, Y. X. Hu and Y. D. Yin, Angew. Chem., Int. Ed., 2007, 46, 7428–7431. 39 C. Huber, I. Klimant, C. Krause, T. Werner, T. Mayr and O. S. Wolfbeis, Fresenius J. Anal. Chem., 2000, 368, 196–202. 40 S. H. Xuan, Y. X. J. Wang, J. C. Yu and K. C. F. Leung, Chem. Mater., 2009, 21, 5079–5087. 41 Y. Sun and H.-F. Wang, J. Chromatogr., A, 2013, 1316, 62–68. 42 C. Kotsmar, K. Y. Yoon, H. Yu, S. Y. Ryoo, J. Barth, S. Shao, M. a. Prodanović, T. E. Milner, S. L. Bryant, C. Huh and K. P. Johnston, Ind. Eng. Chem. Res., 2010, 49, 12435–12443.
This journal is © The Royal Society of Chemistry 2015
