The tuning of metal enhanced fluorescence for sensing applications.

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The tuning of metal enhanced fluorescence for sensing applications† Mainak Ganguly,a Chanchal Mondal,a Joydeep Chowdhury,b Jaya Pal,a Anjali Palc and Tarasankar Pal*a Stable coinage metal nanoparticles (NPs) have been synthesized individually in an aqueous alkaline solution from the corresponding metal salts as precursors using the condensation product (CP) of salicylaldehyde and triethylenetetramine as a reagent. Silver and gold NPs are obtained with and without light illumination but UV irradiation is essential for Cu(0)NP formation. During nanoparticle formation the CP is oxidized to OCP which eventually becomes a fluorophore and also a stabilizer for the in situ produced NPs. It has been observed that silver and gold particle formation kinetics is accelerated by UV exposure. Thus the ease of evolution of coinage metal NP formation relates to their nobility. The as prepared OCP solutions containing coinage metals exhibit a fluorescence contrast behaviour (fluorescence enhancement by Cu and Ag; quenching by AuNP) due to the match and mismatch of wave vectors. The electric field evident from the FDTD simulation abreast of the scattering cross section of the NPs governed from Mie theory as a consequence of surface plasmon coupled emission (SPCE), near field electromagnetic intensity enhancement and lightening rod effect concentrating the electric field around the fluorophore are responsible for the Cu and AgNPs stimulated fluorescence. Again, lossy surface waves are anticipated

Received 19th August 2013, Accepted 1st October 2013

for efficient quenching by the AuNPs. The most unprecedented observation is ‘Turn On’ fluorescence

DOI: 10.1039/c3dt52258j

which is reported here as a result of the substitution of Au(0) or Cu(0) by Ag(0). Finally, the preferential fluorescence enhancement helps the selective detection of Ag(I) and Cu(II) well below the US Environmental

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Protection Agency (EPA) permissible level by tuning the experimental conditions.

Introduction Fluorescence measurement in bulk samples is a very common practice where the solutions are typically transparent to the emitted radiation and isotropic emission of the fluorophore in the free space, which is observed in the far field. An alteration of the refractive index is the main consequence of it. The free space spectral properties of the fluorophore are very minutely influenced with the change of refractive index.1 To explain the huge change of fluorescence near the metallic surface, modifications of the free space conditions of the fluorophore have come into the field during the past decade. The report of Lakowicz et al. reveal that silver island films (SIFs) are interesting candidates for escalating the intensities and photostabilities of fluorophores with low quantum yields.2

a

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Department of Physics, Sammilani Mahavidyalya, Kolkata-700075, India c Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India. E-mail: [email protected]; Fax: (+)91-03222-255303 † Electronic supplementary information (ESI) available: IR, mass, NMR, fluorescence, excitation, absorption and Raman spectra, FESEM image, mechanism of NP formation. See DOI: 10.1039/c3dt52258j b

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SIFs can cause quenching, increased rates of excitation, and/or increased quantum yields depending on the way of interaction with the fluorophore.1 Not only SIFs, but other various silver surfaces like silver colloids,3 silver nanotriangles,4 silver nanorods,5 and fractal-like silver surfaces6 are also found in the context of metal enhanced fluorescence (MEF). One very promising advantage of colloidal suspension over films is the homogeneous distribution of the metals and their possible promising impregnation for medical imaging. But, a lot of interactions in solution make the study complicated. Moreover, to position the metals at the right distance from the fluorophore in solution phase is very difficult. An interesting report by Aslan et al. to produce metal enhanced fluorescence in solution has been found to be a potential sensing platform.7 Again, the Tb3+ complex of salicylic-based open-chain crown ether possesses enhanced fluorescence in silver chloride collosol.8 A study by Gill et al. demonstrates that a dye-DNA composite becomes extremely fluorescent with silver nano aggregates.9 Not only silver, but also gold and copper are known to exhibit MEF. Of course, the number of publications2–6,10 is much higher for silver as it has more favorable imaginary component in the dielectric function than gold and copper. The

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absorption cross section depends on the imaginary component of the metal and it is higher for copper than gold or silver. So, the close-proximity of the fluorophore would cause mostly quenching by the copper nanoparticles. Again, copper also has a scattering component responsible for the MEF effect.11 Localization of surface plasmon resonance (LSPR) at the surface of metal nanoparticles creating an enhancement of the electromagnetic field as well as the coupling between the surface plasmon field of the metal and the molecular dipole of the fluorophore are two mechanisms for the enhancement of fluorescence. According to the radiating plasmon (RP) model, the enhancement as well as the quenching of the fluorophore near the surface of the metal are determined by the optical properties of the metal structures calculated from electrodynamics.12 The extent of the enhancement also lies in the geometry of the metallic nanostructures contributing different surface plasmonic modes at “hotspots”.13 The quenching effect in the proximity of the metal nanoparticles has been ascribed to lossy surface waves (LSWs), dissipated losses, ohmic losses, and such similar phenomena. All of them are members of nonradiative dissipation of energy within metals for the proximity based quenching. The energy of the quenching causing trapped plasmons cannot be recovered as a fruitful signal. A Schiff base [(2-(2′-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine)], CP (the condensation product of salicylaldehyde and triethylenetetramine) is mainly used as a precursor of different metal–ligand coordinations.14 The CP is also known to form mixed valent complexes.15 The structure of the CP and its metal complexes were confirmed from single crystal XRD analysis. The luminescence of dinuclear lanthanide(III)–CP is also reported.16 Lijuan et al.17 have described the fluorescence behaviour of La(III), Ce(IV), Eu(II) complexes of CP. In spite of invasive utility and the broad prospects of sliver in the electronic, photographic and imaging industry,18,19 much attention has been paid to detect Ag(I) ions owing to the high toxicity of silver for aquatic organisms. Again, silver can inactivate sulphydryl enzymes and accumulate in the body.20 Some efforts have been performed by different groups employing fluorescence, UV-visible absorption, atomic absorption, and ICP atomic emission spectroscopy to detect silver ions.21,22 Fluorescent sensors are mostly ready for in vivo and in vitro cellular imaging claiming the superiority of the fluorescence approach compared to other analytical methods. Likewise copper, the third most abundant metal in the human body, plays pivotal roles in many biological processes. Unfortunately, free copper ions are lethal causing cellular toxicity with serious diseases including Alzheimer’s disease,23 Indian childhood cirrhosis (ICC),24 prion disease,25 and Menkes and Wilson diseases.26,27 So, a fluorescent sensor for copper detection becomes a hot research field.28–30 In our present study, a CP has been employed for the first time for the synthesis of coinage metal nanoparticles (MNPs) i.e., copper, silver and gold nanoparticles. As a corollary of synthetic protocol, the as produced silver and copper nanoparticles show a huge enhancement in the fluorescence of the


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in situ oxidized CP (OCP) in their individual capacity and experimental conditions. Surface plasmon coupled emission, near field electromagnetic intensity enhancement and the increasing local incident field around the fluorophore (lightening rod effect) have been ascribed for such an enhancement.31–33 This MEF phenomenon has prompted us to design a highly selective detection method for Cu(II) and Ag(I) with the lower detection limit far below the prescribed level as published by the US Environmental Protection Agency (EPA) [∼460 nM for silver, ∼20 μM for copper]. On the contrary, gold nanoparticles produced in the presence of a CP greatly quench the fluorescence of the OCP in solution due to lossy surface waves.

Experimental section Materials and instruments All of the reagents were of AR grade. Throughout the experiment, triple distilled water was used. Salicylaldehyde, triethylenetetramine (TETA), silver nitrate, chloroauric acid, copper sulphate and dioctyl sulfosuccinate sodium salt (AOT) were obtained from Sigma-Aldrich. NaOH was purchased from HiMedia Laboratories Pvt. Ltd. n-Heptane was purchased from Merck. All glass wares were cleaned with freshly prepared aqua regia, subsequently rinsed with copious amount of distilled water and dried well before use. The sample solution was irradiated with a TUV 15W/G 15 T8 ultra-violet light (Philips India) source. All of the UV-vis absorption spectra were recorded with a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito, India). The FT-IR spectra were recorded with a FT-IR Nexus spectrophotometer (Thermo Nicolet). The 1H-NMR spectrum was obtained with a 400 MHz Bruker NMR instrument. The X-ray photoelectron spectroscopy (XPS) analysis was carried out with a VG Scientific ESCALAB MK II spectrometer (UK) equipped with a Mg Kα excitation source (1253.6 eV) and a five-channeltron detection system. The fluorescence measurement was carried out at room temperature using a LS55 fluorescence spectrometer (Perkin Elmer, USA). The particle morphology was examined using a field emission scanning electron microscope (Supra 40, Carl ZEISS Pvt. Ltd). TEM analysis was performed with an instrument H-9000 NAR, Hitachi, using an accelerating voltage of 300 kV. The Raman spectra of the samples were obtained with a Renishaw Raman microscope, equipped with the laser sources (633, 785, and 514 nm) and a Peltiercooled (∼70 °C) charge-coupled device (CCD) camera. A Leica DMLM microscope was attached and was fitted with three objectives. The synthesis of CP A solution of 2.92 g (20 mmol) TETA in 20 mL of ethanol was added to a solution of 7.32 g (60 mmol) of salicylaldehyde in 10 mL of ethanol with vigorous stirring under ice cold conditions. A bright yellow precipitate was formed after a few hours. The solid product was collected by filtration and dried at room temperature after washing repeatedly with diethyl

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Scheme 1 A reaction scheme indicating the formation of the condensation product (CP) and MNP.

ether. The crude CP (Scheme 1) was recrystallized from methanol. The compound was characterized from melting point (100 °C), elemental analysis (Table S1, ESI†), infrared (Fig. S1, ESI†), mass (Fig. S2, ESI†), 1H-NMR (Fig. S3, ESI†) and 13 C-NMR (Fig. S4, ESI†) analyses. The synthesis of the coinage metal nanoparticles In a 3.2 mL 0.1 M NaOH solution, 0.2 mL 2.5 × 10−3 M CP and 0.15 mL 10−2 M AgNO3 were mixed together with stirring. The CP was dissolved in a 0.1 M NaOH solution as it is insoluble in water. The solution was taken in a fluorescence cuvette and irradiated for one hour to obtain silver nanoparticles (AgNPs). Similarly, gold nanoparticles (AuNPs) were synthesized using HAuCl4 instead of AgNO3 followed by UV exposure for 11 hours. For the preparation of the copper nanoparticles (CuNPs), instead of AgNO3 or HAuCl4, 0.025 mL 10−2 M CuSO4 was introduced and the reaction mixture was exposed to UV light for a period of 14 hours. Computational details The Mie theory calculations have been performed using Mie Calc software.34 Mie theory calculations were performed for the excitation wavelength of 425 nm which corresponds to the emission wavelength of the fluorophore. In the above-mentioned calculations, the complex refractive index (np) for the gold, silver and copper nanoparticles and the real refractive index of the surrounding aqueous medium are taken as 1.45 + i 1.948, 0.04 + i 2.462, 1.25 1 + i 2.305 and 1.331, respectively, for the wavelength of incident radiation (λ) 425 nm. The three dimensional finite difference time domain (3D-FDTD) simulation technique has been utilized to simulate the spatial distribution of the electric field around the silver and copper nanoparticle surfaces. The dispersive properties of the silver and copper nanomaterials have been incorporated in the simulation using the generalized Drude model. According to the generalized Drude model, the complex permittivity ε(ω) is represented as: εðωÞ ¼ ε1 þ

εs ε1 σ þ 1 þ iωτ iωε0

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where εs, ε∞, σ and τ represent the static permittivity, infinite frequency permittivity, conductivity, and relaxation time respectively, ω is the angular frequency, and ε0 is the permittivity of free space. The four parameters εs, ε∞, σ and τ are then adjusted through a curve fitting technique to match the complex permittivity correctly. The complex permittivity ε(ω), as obtained from the generalized Drude model, faithfully represents the optical response of the silver and copper nano materials over a wide frequency range, thereby ensuring the accuracy of the FDTD simulation.35,36 The Yee cell used in the calculation was set at 1 × 1 × 1 nm3 and the step size was fixed at 1.93 atto second. The number of periods of the incident sinusoidal wave was fixed at 10 to ensure the convergence of the calculation. The amplitude of the incident sinusoidal wave was set at 1 V m−1 for the excitation wavelength of 425 nm. The 3D-FDTD calculations were performed using XFDTD 6.3 software (RemCom XFDTD 6.3).

Results and discussion The compound CP, a condensation product of salicylaldehyde and triethylenetetramine (TETA), is known to form complex with only one coinage metal copper, where the ligand to copper ratio becomes 1 : 2.14 We have judiciously selected CP and made proper use of it so that it inculcates dramatic fluorescence behaviour with all of the three coinage metal nanoparticles (NPs) i.e. with Au(0), Ag(0) and Cu(0) NPs (Scheme 1). The CP in our present study is a reagent for the synthesis of AuNPs, AgNPs and CuNPs. The synthetic protocol keeps the Cu(0), Ag(0) and Au(0) nanoparticles in such a position37,38 from the in situ oxidized CP (OCP) that the AgNPs and CuNPs show an enhancement of fluorescence and the AuNPs exhibit quenching (Scheme 2). In this context it should be mentioned that CP is not water-soluble and we have carried out all our experiments in a 0.1 M NaOH solution.

Scheme 2 A schematic representation of the synthesis of coinage metal nanoparticles with the condensation product (CP) of salicylaldehyde and triethylenetetramine (TETA) and then the intriguing fluorescence behaviour of the oxidized CP (OCP) by the in situ produced metal nanoparticles.


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The oxidation of the CP and in turn the reduction of the metal salts take place simultaneously in solution. The phenolic group (λem = 500 nm) of CP is oxidized to its quinone form (λem = 430 nm) which might have the same mechanism as suggested by Selvakannan et al.39 (Scheme S1, ESI†). The mass spectrum (Fig. S1, ESI†) also supports the formation of a quinone (not dimer of CP). The quinone formation is further supported from the 70 nm blue shift and decreased fluorescence due to the loss of conjugation (Fig. 1). Previously we have reported that UV exposure is essential to reduce the metal ions for the formation of nanoparticles in the presence of salen-like molecules.37,38 But in the present case, UV exposure is not vital for the evolution of AgNPs and AuNPs in an alkaline CP solution. However, for the formation of CuNPs, UV exposure is essential due to the comparatively low reduction potential (E0 = +0.34 V) of Cu(II)/Cu(0). Cu(II) needs drastic conditions/a high activation energy which is supplemented by prolonged (14 hours) UV irradiation for the formation of CuNPs. Nevertheless, we have used ∼365 nm UV light deliberately for the preparation of AgNPs and AuNPs also to accelerate the formation kinetics i.e., for time saving issues. Ag(I) needs just one hour under UV light to exhibit a maximum enhancement of fluorescence. But, ∼7 hours of ageing is required to accomplish the same fluorescence intensity for the dark reaction. The case of AuNPs formation is quite interesting. UV exposure for ∼11 hours produces a red colored gold hydrosol with a plasmon band having a fixed λmax at ∼522 nm. But, without UV exposure, a mixture of alkaline CP and a HAuCl4 solution initially produces a brown precipitate containing AuNPs of ∼8 nm average diameter. Then digestive ripening40–42 takes

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place and a monodispersed gold hydrosol is generated with smaller AuNPs (∼5 nm) bringing a tight size distribution profile after 8 days. And, a homogeneous red solution is obtained. Importantly, only UV exposure makes the process of CuNPs nanoparticle formation feasible (Fig. 1). Without irradiation, the fluorescence of the alkaline CP is decreased in presence of Cu(II) due to spin–orbit coupling.37 In the absence of any metal ions, the fluorescence of CP (λem = 500 nm) is destroyed due to photobleaching43 under UV light exposure due to polymerization.44 It is worth mentioning that a peak at ∼430 nm appears due to a significant amount of OCP formation upon UV irradiation of an alkaline CP solution. Finally, both the peaks due to CP and OCP are decreased with time under UV light irradiation. So, the instability of the exposed CP and OCP in the alkaline solution is verified. Fig. 2A represents the kinetics of decrement of the fluorescence intensity of the CP under ∼365 nm UV light exposure. When a mixture of alkaline CP and Ag(I) is irradiated under UV, the fluorescence intensity (λem = 425 nm) of the exposed CP is increased rapidly for one hour indicating the formation of OCP capped AgNPs. Irradiation beyond one hour lowers the fluorescence intensity (Fig. 2B). In the case of the Cu(II) system, UV irradiation results in the gradual appearance of a ∼430 nm peak in the fluorescence spectrum with the decrease of the 500 nm peak (due to CP) like that of the Ag(I) system. After ∼14 hours of UV irradiation, the band at ∼430 nm attains a maximum intensity indicating the quantitative transformation of Cu(II) into the OCP capped CuNPs. Over exposure (irradiation >20 hours) causes little decrease of (Fig. 2C) the CuNP enhanced fluorescence. Under the present situation,

Fig. 1 The formation of a coinage metal hydrosol for fluorescence and TEM studies. (A) The fluorescence intensity of the silver stimulated OCP (oxidized CP) solution after one hour of UV exposure (black), a solution without UV exposure but after ageing the solution for 7 hours (red) and the solution of the CP (without silver) after one hour of UV exposure (blue). (B) The fluorescence intensity of the copper stimulated OCP (oxidized CP) solution obtained after 14 hours of UV exposure (black), a solution without UV exposure but after ageing the solution for 20 hours (red), a solution of the CP (without copper) after 14 hour of UV exposure (green) and the unexposed CP (without copper) solution (blue). (C) TEM images of precipitated Au particles (left) after five hours of mixing of CP/HauCl4 and dispersed Au particles (right) after eight days ageing produced without UV irradiation. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M, [Au(III)] = 4.2 × 10−4 M.


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Fig. 2 The fluorescence spectra of the alkaline (A) CP, (B) CP–Ag(I), (C) CP–Cu(II) solution at different time intervals of UV exposure. (D) The UV-vis spectra of the alkaline CP–Au(III) solution at different time intervals of UV exposure. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M, [Au(III)] = 4.2 × 10−4 M.

over UV exposure kills the silver stimulated MEF (unlike copper stimulated fluorescence) miserably, which is realized from the rapid decrease of the fluorescence intensity of the OCP capped AgNPs. We can successfully monitor the formation kinetics of the AuNPs by UV-vis spectroscopy by measuring the increase of the absorbance value at the λmax (∼522 nm) of the plasmon peak due to AuNP. Thus, the efficient quenching of the fluorescence of OCP by AuNPs is authenticated. After 11 hours of UV irradiation, a stable band in the absorption spectrum appears at ∼522 nm which is not at all perturbed by over time UV irradiation (Fig. 2D). Capping in colloid chemistry is a reversible phenomenon. The ligand stabilizes metal nanoparticles and as a corollary, the ligand gets stabilized by the metal nanoparticles. So, the capping agent (OCP) in turn gets a different extent of stability by the coinage metal nanoparticles under UV light which follows the order: AuNPs > CuNPs > AgNPs Ag(I) needs only one hour of UV irradiation to show a maximum fluorescence intensity, while Cu(II) demands 14 hours of irradiation in the presence of CP. Owing to the huge fluorescence enhancement of OCP in presence of the AgNPs after one hour of UV exposure, we thought the fluorophore CP could be a potential candidate for the Ag(I) sensor. Again, the alkaline CP solution inherits an insignificant fluorescence and the alkaline CP–Cu(II) solution exhibits a decent fluorescence intensity after 14 hours of UV irradiation. So time dependent UV irradiation has paved the way to design a selective Cu(II) sensing strategy. So, we have found the CP to be a

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wonderful compound to behave as a promising Ag(I) and Cu(II) sensing platform just by fine-tuning the experimental conditions. To examine the selective Ag(I) and Cu(II) sensing applications in their respective reaction conditions, we have introduced other different metal ions in lieu of Ag(I)/Cu(II). We have found that the fluorescence enhancement of the OCP by silver and copper is quite discriminating in their particular reaction conditions in comparison to other metal ions (Fig. 3). The ratio of Ag(I) or Cu(II) to the CP concentration is very important in the context of MEF. When the CP concentration is higher, to obtain maximum silver enhanced fluorescence the Ag(I) ion concentration should be ∼3 times that of the CP concentration. But, with the decrease of the CP concentration, the Ag(I) : CP increases to have a maximum fluorescence enhancement. For trace level detection of Ag(I) in solution down to a very low concentration level, the CP concentration should also be as low as possible. At a low CP concentration, silver enhanced fluorescence is found to be pronounced even at a remarkably low Ag(I) precursor concentration. Employing this idea in mind we have detected Ag(I) as low as 9 × 10−8 M when the CP concentration is kept at 1.4 × 10−6 M. The result comes from seven replicate measurements. Fig. 4 represents the fluorescence spectral profile of the Ag(I) concentration dependent silver enhanced fluorescence at different CP concentrations. The results have also been summarized in the table denoting that the LOD (lower detection limit) is increased with the decrement of [CP]. On the contrary, the linear detection range and (I/I0)max [I and I0 are fluorescence intensity with and without metal] are enhanced with the increase of [CP].


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Fig. 3 (A) A bar diagram indicating the fluorescence intensity with different metal ions used individually under the optimum condition where silver causes maximum MEF and the inset indicates the normalized fluorescence spectra. (B) A bar diagram indicating the fluorescence intensity with the variation of metal ions at the optimum condition where copper causes maximum MEF and the inset indicates the normalized fluorescence spectra. Conditions for (A): [CP] = 1.4 × 10−4 M, [Mn+] = 4.2 × 10−4 M, exposure time = one hour, λex = 290 nm. Conditions for (B): [CP] = 1.4 × 10−5 M, [Mn+] = 0.70 × 10−5 M, exposure time = 14 hours, λex = 290 nm.

Similar experiments are performed for Cu(II) in the appropriate conditions for copper enhanced fluorescence. A similar type of observation is noticed regarding the LOD and linear detection range like silver. But, for the case of copper, the 2 : 1 = CP : Cu(II) produces the highest degree of enhancement whatever the concentration of CP may be. It implies the 2 : 1 complex formation of CP with the Cu(II) before the formation of Cu(0). Seven replicate measurements with copper indicates that we can go down to 9 × 10−8 M Cu(II) concentration when the CP concentration is 1.4 × 10−6 M (Fig. 5). The maximum copper stimulated fluorescence, (I/I0)max is obtained with 1.4 × 10−5 M CP and 7.15 × 10−6 M Cu(II) after ∼14 hours irradiation. Selective detection is achieved easily from short and long times of UV exposure. Ag(I) and Cu(II) are detected selectively through one hour and 14 hours of UV exposure respectively.


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Under appropriate conditions Cu(0) and Ag(0) exhibit 166 and 52 times enhancement of fluorescence. To observe the highest degree of selective fluorescence enhancement in the presence of AgNPs and CuNPs, the choice of excitation energy is crucial. The excitation energy for OCP capped AgNPs and CuNPs is 290 and 300 nm respectively to observe a maximum fluorescence intensity. To make the fluorescence process more selective as well as sensitive, we have chosen 290 nm as the excitation energy. Under these circumstances, we get maximum I/I0 [I0 = fluorescence intensity of OCP; I = fluorescence intensity of OCP along with in situ produced metal NPs] for both the systems (Fig. 6). It is to be noted that a sharp band with a λmax at ∼290 nm is observed in the excitation spectra which is different from the absorption spectra indicating a different interaction in the excited state for both the copper and silver systems (Fig. S6 and S7, ESI†). The time evolution fluorescence spectroscopy reveals a single lifetime component (3.8 ns) of the AgNPs capped fluorophore indicating that all the molecules are in the OCP form. The CuNPs capped fluorophore has one slow (3.9 ns, 47%) and one fast component (1.52 ns, 53%) indicating that the solution contains both OCP and CP. Again, when the CP is irradiated, both the OCP and CP are formed exhibiting the biexponential (1.19 and 3.70 ns for fast and slow component) nature of the life time plot (Fig. 7). We have mentioned earlier that the fluorescence induced by AgNPs is not stable enough in comparison to the gold and copper systems under prolong UV exposure. Surprisingly, after the formation of a gold or copper hydrosol from an individual reaction mixture, the addition of Ag(I)NO3 causes ‘Turn On’ fluorescence. It is due to the formation of AgNPs and the simultaneous expulsion of AuNPs or CuNPs from the solution. As a result of the substitution by Ag(0), a precipitate due to expelled Au(0) or Cu(0) from their respective hydrosol system is observed rendering Ag(0) enhanced fluorescence. In this case the birth of the fluorescence intensity is to some extent lower than the pure silver hydrosol case. This is due to the removal of some bound OCP along with AuNPs or CuNPs as a precipitate. However, the maximum recovery of the Ag(0) enhanced fluorescence is observed when Ag(I) is introduced to the red colored gold hydrosol. This is evidenced from the appearance of a fluorescent yellow solution which supports Ag(0) enhanced fluorescence conclusively. Besides, a black precipitate containing Au(0) particles of average diameter ∼50 nm (Fig. S8, ESI†) is thrown out of the solution. Similar Ag(0) substitution experiments have been done with other metal ions also. Interestingly, we have found solutions with a variable fluorescence intensity as depicted in Fig. 8. But for Co, Pd, Pt and Hg, no fluorescence is recovered due to their association with Ag. Similarly, CP has shown promise to design Cu(II) sensor because after 14 hours of UV exposure, no metal produces such fluorescence enhancement like Cu(II) as shown earlier. A maximum enhancement of fluorescence is observed when CP : Mn+ = 2 : 1 ratio. To observe the effect of the interfering metal ions, we have repeated the experiment with other metal ions along with Cu(II). The photoproduced CuNPs enhanced

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Fig. 4 Normalized fluorescence spectra (A1–D1) of OCP (oxidized CP after one hr UV irradiation) with the variation of [Ag(I)] at different CP concentrations using 290 nm excitation; (A1) [CP] = 2.8 × 10−4 M, (B1) [CP] = 1.4 × 10−4 M, (C1) [CP] = 1.4 × 10−5 M and (D1) [CP] = 1.4 × 10−6 M. The degree of fluorescence enhancement (A2–D2) with the variation of [Ag(I)] at different CP concentrations and the inset represents the linear detection range; (A2) [CP] = 2.8 × 10−4 M, (B2) [CP] = 1.4 × 10−4 M, (C2) [CP] = 1.4 × 10−5 M and (D2) [CP] = 1.4 × 10−6 M. The Table represents the linear detection range, lower detection limit, (I/I0)max, Ag(I) conc. for (I/I0)max.

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Fig. 5 Normalized fluorescence spectra (A1–D1) of OCP (oxidized CP after 14 h UV irradiation) with the variation of [Cu(II)] at different CP concentrations using 290 nm excitation; (A1) [CP] = 2.8 × 10−4 M, (B1) [CP] = 1.4 × 10−4 M, (C1) [CP] = 1.4 × 10−5 M and (D1) [CP] = 1.4 × 10−6 M. The degree of fluorescent enhancement (A2–D2) with the variation of [Cu(II)] at different CP concentrations and the inset represents the linear detection range; (A2) [CP] = 2.8 × 10−4 M, (B2) [CP] = 1.4 × 10−4 M, (C2) [CP] = 1.4 × 10−5 M and (D2) [CP] = 1.4 × 10−6 M. The Table represents the linear detection range, lower detection limit, (I/I0)max, Cu(II) conc. for (I/I0)max.


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Fig. 6 The silver (A1) and copper (B1) stimulated fluorescence spectra of OCP (normalized) with the variation of excitation wavelength. Both insets represent the corresponding fluorescence spectra of OCP (normalized) without metal but with variations of the excitation wavelength; A2 and B2 represent the corresponding I/I0 with the variation of excitation wavelength. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, exposure time = one hour for (A1) and (A2) i.e., for silver. [CP] = 1.4 × 10−5 M, [Cu(II)] = 0.70 × 10−5 M, exposure time = 14 hours for (B1) and (B2) i.e., for copper.

Fig. 7 The life time measurement of the exposed CP (UV exposure time one hour), CP–Ag(I) (UV exposure time one hour) and CP–Cu(II) (UV exposure time 14 hours). Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M.

fluorescence is not affected in the presence of other interfering metal ions (Fig. S9, ESI†). If we keep the OCP capped AgNPs [produced photochemically from alkaline CP by one hour exposure or Ag(I) in CP solution (unexposed) kept for 7 hours] in ambient (25 °C) conditions, we find that the fluorescence intensity is increased successively for 8 days. After that the solution remains stable for >60 days without any alteration in the fluorescence intensity. The successive increase of the fluorescence intensity is due to the slow precipitation of extra silver(0) as a brown precipitate [authenticated from XPS45 and powder XRD46 analysis

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Fig. 8 A bar diagram indicating the fluorescence intensity (measured after seven hours) of the reaction mixtures and normalized fluorescence spectra (inset) after substitution of metal ions by silver. Conditions: different metal ions in alkaline CP are separately exposed for one hour under UV and then Ag(I) is introduced. Fluorescence measurements are done after seven hours of waiting; [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [other Mn+] = 4.2 × 10−4 M, λex = 290 nm.

with the precipitate]. The time dependent fluorescence increment of the solution is presumably related to the removal of Ag(0) from the quenching zone1 of the fluorophore OCP and the further aggregation of the silver particles as reported by Lukomska et al.47 But for the copper system, the fluorescence


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Fig. 9 (A) Powder XRD, (B) the XPS spectra for the element silver of the precipitate from the silver hydrosol after 8 days of ageing. (C) The normalized fluorescence spectra and (D) TEM studies to investigate the effect of aging on the silver hydrosol. (E) The TEM image of the freshly prepared photoproduced CuNPs and (F) the unaltered CuNP stimulated fluorescence with ageing. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M, λex = 290 nm.

intensity does not change on ageing unlike the silver system (Fig. 9). To investigate the donor capability of the capping agent, we have carried out the experiment in D2O. The 1H-NMR (Fig. S10, ESI†) spectrum reveals that in an alkaline D2O solution, the peak at 9.8 ppm due to the –CHvN– proton45 becomes acidic after the introduction of Ag(I) and is keep for 7 hours. So, the peak vanishes after being exchanged with D2O. This observation proves that the electron density of the iminic bond is involved in capping the coinage metal nanoparticles for long-term stability with interesting fluorescence behaviours. Again, the metal–nitrogen interaction along with the quinone moiety has been substantiated from Raman spectroscopy (Fig. S11, ESI†). Thus the long term stability of metal NPs in CP the solution is due to the tight capping by the iminic bond. Whatever the concentration of Au(III) introduced to the alkaline CP solution, we have always found the quenching of fluorescence of OCP ( photoproduced) by the highly stable (under ambient conditions) AuNPs. With the increase of the precursor HAuCl4 concentration, the plasmon band (λmax 522 nm) due to the formation of the AuNPs gradually increases without any shift of the band. I0/I exponentially (the Stern–Volmer plot) grows with respect to [Au(0)] indicating both static and dynamic quenching by the AuNPs. It should be remembered that with the increase of the precursor Au(III) concentration, more OCP is formed as a corollary of evolution of more AuNPs. Again, there is the factor of photobleaching of OCP and CP under UV irradiation. So, at least it can be understood that the I0/I vs. Au(III) concentration would be modified to some extent if the above points are taken into account (Fig. 10). XPS measurements of the hydrosol under freeze drying conditions imply the zero oxidation state of the as prepared


coinage metal nanoparticles in the presence of CP. The binding energy of 366.21 eV and 372.16 eV represents Ag(0) 3d5/2 and Ag(0)3d3/2 respectively.38,48 The shift of the peak toward a lower binding energy for Ag(0) signifies tight capping by the OCP. Similarly, the 932.74 eV and 952.77 eV binding energy symbolizes Cu(0)2p3/2 and Cu(0)2p1/2.37 The lattice fringes of the AgNPs and CuNPs are obtained as 0.234 and 0.211 nm respectively [for (111) plane] supporting their zero oxidation state. The XPS spectra (binding energy of 81.58 eV and 85.19 eV corresponds to Au(0)4f7/2 and Au(0)4f5/2 respectively) and lattice fringe (0.236 nm for (111) plane) indicate the zero oxidation state of gold (Fig. 11).38 The OCP molecule near the copper and silver nanoparticles is not associated with the dramatic life time change in comparison to the OCP molecule in the absence of the metal. In order to understand the enhancement of fluorescence, the surface plasmon-coupled emission (SPCE) model as proposed by Lakowicz31 has been considered. According to the SPCE model, plasmons are created when the excited-state fluorophore interacts with the nearby metal nanoparticle. The plasmons thus generated then radiate or are trapped into the substrate depending upon the scattering and absorption cross section of the nanoparticle. The plasmon radiation is generalized as the radiating plasmon (RP) model. In order to estimate the scattering (QSca) and absorption efficiencies (QAbs) of the copper, silver and gold nanoparticles, Mie theory49 has been successfully applied. When an isolated spherical particle is illuminated by electromagnetic radiation of a given wavelength, Mie theory49 helps one to estimate the scattering (QSca) and extinction (QExt) efficiencies where extinction is the removal of light from the incident beam by absorption and scattering.

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Fig. 10 (A) The absorption spectra, (B1) fluorescence spectra and (B2) I/I0 with the variation of precursor Au(III) concentration. Conditions: [CP] = 1.4 × 10−4 M, exposure time = 11 hours.

According to Mie theory, the scattering (QSca), absorption (QAbs) and extinction (QExt) efficiencies are expressed as: QSca ¼

1 2X ð2n þ 1Þðjan j2 þ jbn j2 Þ x2 n¼1

QExt ¼

1 2X ð2n þ 1Þ½Reðan þ bn Þ 2 x n¼1

QAbs ¼ QExt QSca where n is the summation index of the partial waves; n = 1, 2, 3 correspond to the dipole, quadrupole and octapole oscillations, an and bn are the Mie coefficients which can be evaluated from the complex Bessel–Riccatti functions as reported elsewhere.50 Fig. 12A shows the variations of QSca, QAbs and QExt with the wavelength (λ) for the copper, silver and gold nanoparticle having radii of ∼50, 100 and 2.5 nm, respectively. For the copper and silver nanoparticles, the scattering efficiencies (QSca) are quite appreciable particularly in the excitation wavelength region which corresponds to the intense fluorophore emission (∼425 nm) as explained by Lakowicz.12 However, for the gold nanoparticle, due to its reduced size (diameter ∼5 nm), the scattering efficiency (QSca) is almost zero and the entire absorption (QAbs) efficiency corresponds to the total extinction cross section (QExt). For the gold nanoparticle, the increase in absorption efficiency (Fig. 12A(a3)) may result in the oscillations of those

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electron clouds for which wave vector matching is not possible and the energy is dissipated as heat. This may result in the quenching of the fluorescence of the molecule when adsorbed on gold nanoparticles. However, the size and hence the diameters of the copper and silver nanoparticles are much larger than that of gold. Though the fluorescence of the molecule in the vicinity of both on the copper and silver nanoparticles is significantly enhanced, their degree of enhancement is not the same. For example the fluorescence enhancement is ∼52 fold for silver while for copper it is only ∼5 fold if 1.4 × 10−4 M CP is employed. In order to envisage the origin for the variations in the fluorescent enhancement factors for the silver and copper nanoparticle, 3D-FDTD simulation studies have been performed. The snapshots indicating the spatial distributions of the electric field around the silver and copper nanoparticles are shown in Fig. 12B. A significant ten fold enhancement of the electric field around the silver nanoparticle has been estimated, while for copper, the amplitude of the electric field remains unperturbed. The enhancement of the electric field around the silver nanoparticle upon excitation with the fluorophore emission (∼425 nm) wavelength in addition to the SPCE may result in the increase in the fluorescence enhancement of molecule adsorbed on the silver nanoparticle. To observe a maximum enhancement the concentration of copper is kept 4 times less than that of silver. In other words, CP : Cu(II) = 2 : 1 exhibits a maximum fluorescence enhancement indicating the formation of a chelate followed by the evolution of the CuNPs due to photoirradiation. A computational


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Fig. 11 HRTEM images of the (A) gold, (B) silver and (C) copper hydrosol. The XPS spectra of the (A1) gold, (B1) silver and (C1) copper hydrosol. Conditions: [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, [Cu(II)] = 0.70 × 10−4 M, [Au(III)] = 4.2 × 10−4 M.

Fig. 12 (A) The extinction, scattering and absorption cross section of (a1) 200 nm silver, (a2) 100 nm copper and (a3) 5 nm gold sphere. (B) Snapshots indicating the near field distribution around (b1) 200 nm silver sphere and (b2) 100 nm copper sphere obtained from a 3D-FDTD simulation.

study involving AuNPs and AgNPs agrees well with our experimental fact. But the fluorescence enhancement, which we observed for the CuNPs in the prescribed condition, is not reflected in theory i.e., the FDTD simulated result. It is to be noted in this context of theoretical study that we have assumed a spherical particle geometry. Our experimental results speak for a much improved fluorescence that may be due to


highly aggregated particle assembly and not accommodated in theory. As a corollary of SPCE, the enhancement of the electric field around the metal may play a significant role. But, concentrating the field around OCP i.e., the lightening rod effect may also be of great importance.1 It is indeed true that the nearfield electromagnetic intensity enhancement is considered as the major mechanism for fluorescence enhancement by metal

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nanoparticles. The enhancement of the fluorescence intensity of the probe molecules in the proximity of the metal nanoparticles may not be exclusively considered to be due to the plasmon of the metal nanoparticles alone. It may be due to the radiation from the plasmon–fluorophore association generated from the interaction between the excited fluorophores and the metal nanoparticles in their immediate proximity (near-field). The radiating fluorophore is considered as an oscillating point dipole. According to the radiating plasmon model51–53 the enhanced fluorescence emission observed in the MEF experiments is due to the radiation from the entire excited state fluorophore–metal nanoparticle behaving as a single radiating entity. However, in the present manuscript, we have simulated the near electric field distribution around the metallic nanoparticles (Fig. 12B) arising from the localized surface plasmon resonance (LSPR). The interaction of the near electric field emitting from the fluorophore with that of the plasmon of the nanoparticle could not be presented due to the limitations of the software (XFDTD 6.5) used for the 3D-FDTD simulation. We have disclosed in this piece of work a unique fluorescence behaviour from a known compound in solution. It has been efficiently used to detect Ag(I) and Cu(II) ions using MEF phenomena. Except for Cu(II), no other coinage metal requires any UV exposure unlike our previous report for the reduction to zero state.34,35 The quenching of the fluorescence of OCP can be explained by the damping of dipole oscillations by the nearby field.1,54 The “lossy surface waves” originated by the AuNPs quench fluorescence as induced electron oscillations cannot radiate to the far-field because of wave vector mismatch and optical constraints at the metal–sample interface. Again, concentrating the local incident field55 on the fluorophore causes an increment of the rate of excitation with enhanced fluorescence intensity which is called the lightening rod effect. The proximal metal can also enhance the radiating decay rate (at which the fluorophore emits photons). As a consequence, the increase of fluorescence intensity and decrease of the life time are observed. SPCE is also pointed out by several groups for enhancement.12 In our case, no virtual change of lifetime of the fluorophore is observed for the case of the CuNPs and AgNPs stimulated fluorescence eliminating the probability of modification of radiative decay rate. Here, SPCE, the near field electromagnetic intensity enhancement and the lightening rod effect are responsible for the observed fluorescence enhancement by copper and silver in solution. Piercing a small hole 0.5 mm in diameter in the sharp end of a chicken egg, we have removed the yolk and egg white. The rest of the hard egg wall along with the internal semipermeable membrane is kept intact. The interior of the emptied egg membrane is repeatedly washed by running triple distilled water. Then two thirds of the outer hard CaCO3 casing from the blunt end is dissolved by dilute HCl to expose the internal semipermeable membrane to perform diffusion experiments. We poured 6 mL 2.8 × 10−4 M AgNO3 into the cavity of the egg and kept the egg membrane with

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the remaining hard casing in a beaker containing 32 mL of 1.4 × 10−4 M aqueous alkaline CP solution (set 1, amount of CP is much more than silver). In another set of experiments, the egg casing with the membrane is placed in a beaker. In that set, 6 mL of the CP solution (1.4 × 10−4 M) is taken inside the cavity of the egg and 32 mL AgNO3 (2.8 × 10−4 M) solution is taken in the beaker (set 2, amount of silver is much more than CP). In both of these cases the solutions (inside the cavity and in the beaker) maintain an equal height for the equalization of hydrostatic pressure. After 3 hours, it has been found that the colourless silver nitrate solution (internal solution in the egg membrane, set 1 and external solution set 2, outside the egg membrane) becomes blackish brown due to the formation of silver hydroxide. After ageing for 2 days, it is observed that the brown solution inside the egg of set 1 depicts a higher fluorescence and a pale yellow solution outside the membrane shows a lower fluorescence intensity. For set 2, the solution outside the membrane becomes dark brown with no fluorescence (due to very high silver concentration) and the solution inside the membrane exhibits a lower fluorescent intensity. This indicates that the alkaline CP diffuses more efficiently through the egg membrane than the silver ion (Fig. 13). More elaborately, the alkaline CP (solution in the beaker in set 1) gets hold of a small amount of silver that diffuses out and exhibits a low fluorescence intensity. Again, a smaller amount of the CP solution (set 2) exhibits a low fluorescence intensity due to its interaction with trace silver due to the slow silver diffusion across the membrane. A highly intense fluorescence is observed for the solution inside the egg membrane (set 1) indicating the easy passage of the CP from the huge supply through the membrane into the egg. It is worth noting that the easy passage of the alkaline CP (supply is less) could not bring an intense fluorescing solution in the beaker (set 2) because of the quenching due to the presence of a large amount of silver. This experiment broadens the applicability of the silver enhanced fluorescence study in vivo. Trace amounts of silver if present in the cell can be detected easily with the help of the CP exploiting its unique metal enhanced fluorescence capability. The environment has a vital role to produce lossy trapped plasmons, SPCE and the lightening rod effect. In other words, the fluorescence quenching and enhancement are guided by not only the fluorophore and metal ions but also by the surroundings. To understand this fact, we have performed the reactions in the water pool of the reverse micelle.56 We have found quenching (I/I0 = 0.23 in the water pool of the reverse micelle and I/I0 = 52 in aqueous alkali) and insignificant enhancement (I/I0 = 1.16 in the water pool of the reverse micelle and I/I0 = 121 in aqueous alkali) of the fluorescence for brown silver sol and yellow copper sol respectively when they are generated in the water/n-heptane/ 0.1 M AOT (R = 40) reverse micelle instead of water as mentioned in the Experimental section. Again, the red colored gold sol with an efficient quenching (I/I0 = 0.25 in the water


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Fig. 13 (A) A schematic representation of the permeation experiment indicating the facile movement of the alkaline CP through the egg membrane. (B) The absorption and (C) fluorescence spectra of the solutions in and outside the cavity of the egg after 2 days.

pool of the reverse micelle and I/I0 = 0.12 in aqueous alkali) capability is produced in water/n-heptane/0.1 M AOT microemulsions (Fig. 14).

Conclusions A large fluorescence enhancement of the solution containing OCP capped Cu or AgNPs ( photoproduced) proves an intriguing phenomenon to study. Furthermore, OCP capped Cu and AuNPs are selectively replaced by incoming AgNPs producing the respective precipitate and enhanced fluorescence. Again, the efficient fluorescence quenching of OCP by in situ generated gold nanoparticles is a notable contrast which makes the study more interesting. Thus MEF (metal enhanced fluorescence) helps to configure selective Cu(II) and Ag(I) sensing just by the judicious manipulation of the reaction conditions. The fluorophore may be proved to be worthy of biomedical applications due to its selectivity regarding the fluorescence behaviour involving coinage metals. Further study is


Fig. 14 The fluorescence spectra of gold (ai), silver (bi) and copper (ci) systems; i = 1 stands for CP in aqueous alkaline medium and i = 2 stands for CP in the water pool of water/heptane/AOT micro reactors. Conditions: [CP] = 1.4 × 10−4 M, [Ag(III)] = 4.2 × 10−4 M, exposure time = 11 hours for (ai). [CP] = 1.4 × 10−4 M, [Ag(I)] = 4.2 × 10−4 M, exposure time = one hour for (bi). [CP] = 1.4 × 10−5 M, [Cu(II)] = 0.70 × 10−5 M, exposure time = 14 hours for (ci).

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warranted regarding the matching of the wave vector to understand the fluorescence enhancement/quenching mechanisms.


Acknowledgements The authors are thankful to the UGC, DST, NST, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance. The authors are also thankful to Professor Anindya Datta, IIT Bombay, India for life time measurements.

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