CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201402534
A Fluorescent and Colorimetric Sensor for Nanomolar Detection of Co2 + in Water Anil Kuwar,*[a] Rahul Patil,[a] Amanpreet Singh,[b] Ratnamala Bendre,[a] and Narinder Singh*[b] A new disulfide-based, imine-linked fluorescent receptor 1 was processed into organic nanoparticles (ONPs) with an average particle size of 79 nm. The photophysical properties of the ONPs were evaluated by UV/Vis absorption spectroscopy. Receptor 1 selectively recognized Co2 + ions in water with a detection limit down to 88 nm.
Organic nanoparticles (ONPs) prepared from a variety of materials, including natural products, polymers and biomolecules, have found exciting applications in materials and biological sciences.[1–5] With respect to inorganic nanoparticles, ONPs are anticipated to allow much more structural changeability through well-known synthetic strategies. Moreover, ONPs dispersed in water validated the utilize of probes in drug delivery system (DDS) and cellular imaging.[6] Singh et al. have reported systematic work on ONPs with the use of Biginelli-Based and other Schiff base nanoparticles that disclosed the size-dependent fluorescent properties of organic materials.[7] For biological and chemical analysis, the probe should be soluble in pure water; however most of the chemosensors are hydrophobic so freshly an innovative concept has been created, which include with the synthesis of ONPs.[8] In sensor development, the ratiometric fluorescence determination of the analyte has great advantages over the conventional monitoring of fluorescence intensity at a single wavelength. The detection of Co2 + among the transition-metal ions has attracted much attention because cobalt is one of the most important trace elements in human beings. In the structure of vitamin B12 (cobalamin), this metal takes part in a number of critical tasks in many biological purposes.[9] Cobalamin is essential for DNA synthesis, the formation of red blood cells, the continuation of the nervous system, and the growth and expansion of children.[10] Similar to other critical elements, cobalt can be less toxic than non-vital metals like platinum.[11] However, because of the crucial biological role of cobalt throughout the metabolism of mammals, its imbalance may lead to diseases and malignancies.[12] The excess of cobalt from the normal permissible limit can cause detrimental effects such as vasodi[a] Dr. A. Kuwar, R. Patil, Prof. R. Bendre School of Chemical Sciences, North Maharashtra University Jalgaon—425001 MS (India) E-mail: [email protected] [email protected] [b] A. Singh, Dr. N. Singh Department of Chemistry, Indian Institute Technology Roapr-140001 Punjab (India) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402534.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
latation, flushing and cardiomyopathy, whereas its deficiency in humans and animals results in anemia.[13] Because of the potential need for a highly sensitive analytical method, there is immense interest in the development of Co2 + -selective chemosensors for various biological and environmental applications.[14] Our current research work is focused on the synthesis of fluorescence-based organic nanoparticles for the sensitive and selective detection of particular cations over other cations in water. In this assembly, sulfur, imine, and hydroxyl groups are bearing responsible binding sites for cation recognition. Herein, we explored disulfide–imine-based ONPs (1) as a highly selective “turn-on” fluorescence sensor for Co2 + ions.
Experimental Section All spectroscopy-grade chemicals and solvents were purchased from Aldrich and used without further purification. Fluorescence and UV/Vis spectra were recorded on a Fluoromax-4 Spectrofluorometer and a Shimadzu UV-24500 equipment, respectively, with a 5 nm slit of fluorescence; the nitrate salts of the metal were used in the study. Ultrapure water obtained with a Millipore Purification System (Milli-Q water) was used throughout the analytical experiments. 1H NMR spectra were recorded on a Jeol spectrometer operating at 400 MHz in [D6]DMSO.
Synthesis of Receptor 1 Receptor 1 was synthesized by reacting one mole of 2-(2-(2-aminophenyl)disulfanyl)benzenamine (0.24 g, 1 mm) with one mole of 2hydroxy-3-isopropyl-6-methylbenzaldehyde (0.17 g, 1 mm) in ethanol, with stirring and refluxing for three hours at ambient temperature. Receptor 1 was obtained in good yield as a yellow powder. Yield: 86 %, m.p. 156 8C. 1H NMR (400 MHz, [D6]DMSO, ppm) d = 1.22 (d, 12 H, gem 4 CH3), 2.25 (s, 6 H, Ar-CH3), 2.85 (septet, 2 H, CH), 6.98(d, 2 H, Ar-H), 7.06(d, 2 H, Ar-H), 7.10–7.32 (m, 8 H, Ar-H), 8.55 (s, 2 H, -CH = N), 11.09 (s, 2 H, -OH). 13C NMR (100 MHz, [D6]DMSO, ppm) d = 15.93, 24.12, 29.08, 113.89, 114.87, 114.98, 124.97, 126.53, 127, 130.91, 131.26, 141.16, 142.79, 143.93, 156.23, 166.36; CHN analysis: Calculated = C 71.79, H 6.38, N 4.93. Obtained = C 71.64, H 6.18, N 4.69.
Synthesis of the Organic Nanoparticles (ONPs) The ONPs were obtained by the reprecipitation method,[15] in which a solution of the organic compound is injected into water and the resulting organic nanoparticles are obtained as a result of precipitation. This method is simple in operation and the particles obtained are usually about ten to hundreds of nanometers in diameter. Briefly, for the preparation of the ONPs, 5 mg of receptor 1 was first dissolved in a minimum amount of dimethyl sulfoxide (DMSO, 1 mL). This receptor solution was taken in a syringe and inChemPhysChem 2014, 15, 3933 – 3937
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of the organic compound is injected into water and the resulting organic nanoparticles are obtained as a result of precipitation. This involves the injection of the organic compound [1 mL of a stock solution of 1(ONPs) in pure DMSO] to 100 mL of distilled water. There is great disparity between the solubility of 1(ONPs) in DMSO (good solubility) and water (highly insoluble); however, the good solubility/ miscibility compatibility of the two solvents is the governing feature of the method.[16] Sonication ensured the rapid mixing of both the solutions. Receptor 1 is exposed to water for a very short time, and the water is expected to change the microenvironment of 1(ONPs) inducing the nucleation and growth of molecules to nanoparticles. The particle-size distribution of the nano-particles was estimatFigure 1. a) Particle-size analysis of 1(ONPs), measured by DLS. b) TEM analysis showing the particle size of 1(ONPs). c) Comparison of the UV/Vis absorption spectra of ligand 1 (red line) and 1(ONPs) (blue line). ed by DLS; the studies revealed that the size of the 1(ONPs) was in the range from 50 to 120 nm, approximately, whereas the average size of the 1(ONPs) was found to jected to 100 mL of distilled water in small increments under sonibe 79 nm (Figure 1 a). The spherical shape of the 1(ONPs) was cation, which was continued for about half an hour to ensure the demonstrated by TEM analysis, as ashown in Figure 1 b. An assesspreparation of stabilized ONPs; these materials are denoted as ment of the UV/Vis absorption spectra of 1 (recorded in pure 1(ONPs) throughout the manuscript. During the course of solution DMSO) with 1(ONPs) (recorded in water) revealed changes in the mixing, the size of the nanoaggregates was analyzed by DLS (difphotophysical properties on aggregation of the material.[17] From ferential light scattering) and was found to be 79 nm, Moreover, the comparison of the UV/Vis absorption spectra of receptor 1 and TEM (transmission electron microscopy) images were taken to 1(ONPs), it is concluded that the excitation band for receptor check the uniformity of the nanoaggregates (Figure 1 b). The TEM 1 occurs at 345 nm, whereas for 1(ONPs) the band occurs at analysis revealed that the particles are uniformly distributed and 285 nm, and moreover, the band for receptor 1 broadens after the have sizes smaller than 70 nanometer. formation of 1(ONPs) (Figure 1 c). Initially, the colorimetric detection ability of receptor 1(ONPs) was studied by treating it with variUV/Vis and Fluorescence Spectral Measurements ous cations (2 equivalents). It was observed that the color of the receptor changes from colorless to brown only in the case of Co2 + The metal ions Hg2 + , Pb2 + , Mn2 + , Co2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , salts. The color change could be detected by the naked eye Fe2 + , Fe3 + and Cd2 + were added as their nitrate salts for absorp(Figure 2 inset). However, the addition of other cations did not tion and fluorescence spectroscopic experiments. Stock solutions result in detectable color changes. 3 4 of the metal ions (1 10 m) and the receptor 1(ONPs) (1 10 m) were prepared in water. These stock solutions were used after apThe changes in the UV/Vis spectra of 1(ONPs) (10 mm), caused by propriate dilution. Co2 + (10 equiv) and miscellaneous cations (10 equiv) including
Results and Discussion Receptor 1 was synthesized by a condensation reaction between 2-(2-(2-aminophenyl)disulfanyl)benzenamine and one mole of 2-hydroxy-3-isopropyl-6-methylbenzaldehyde in ethanol (Scheme 1). The condensation product 1 was characterized by using spectroscopic methods, that is, the formation of CH = N- bond was established from NMR; while elemental analysis (CHN) gave the correct structural formula of the product. ONPs of receptor 1(ONPs) were obtained by the the re-precipitation method,[15] in which a solution Scheme 1. Synthesis of receptor 1. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
alkali, alkaline earth, and transition-metal ions, Hg2 + , Pb2 + , Mn2 + , Co2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , Fe2 + , Fe3 + and Cd2 + in water solution, are recorded in Figure 2. The various alkali, alkaline earth, and transition metal ions did not induce any apparent absorption changes, even upon addition of 10 equiv of the respective metal ions. Addi-
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Figure 2. UV/Vis spectra of 1(ONPs) (10 mm) in water in the presence 10 equiv of Co2 + ions and other cations including Hg2 + , Pb2 + , Mn2 + , Co2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , Fe2 + , Fe3 + , and Cd2 + . The inset shows the visual change in the solution of receptor 1(ONPs) upon addition of Co2 + . Left: receptor 1(ONPs); right: receptor 1(ONPs) + Co2 + .
tion of 10 equiv of Co2 + to a solution of 1(ONPs) immediately resulted in a significant red-shift of the absorbance from 357 to 457 nm, as well as a color change of the solution from colorless to brown. These facts suggested that the 1(ONPs) behaved as an efficient colorimetric probe for the detection of Co2 + ions owing to the fact that the color changes can be easily distinguished by the eye (Figure 2 inset). In the UV/Vis titration of Co2 + ions, the 1(ONPs) showed characteristic absorption peaks at 357 and 457 nm. Upon treatment with Co2 + ions, the absorption peak at 357 nm decreased whereas that at 457 nm increased, highlighting the occurrence of an isosbestic point at 395 nm (Figure 3). The anion recognition properties of 1(ONPs) have also been investigated by monitoring the change in the fluorescence spectra upon the addition of different tetrabutylammonium salts of different anions to the host solution. A solution of 1(ONPs) (10 mm) was mixed with aliquots of different anions (20 equivalent) such as F , Cl , Br , I , CH3COO , NO3 , H2PO4 , HSO4 and ClO4 . All photoluminescence measurements were performed under the same conditions as those used to study cation recognition. No significant changes in the fluorescence spectra were observed upon addition of any of the tested anions (Figure S1 of the Supporting Information, SI). To obtain further insights into the Co2 + sensing properties of 1(ONPs), the emission was examined with various metal species
Figure 3. Spectra taken during the titration of 1 (ONPs) (10 mm) in water with a standard solution of Co2 + at room temperature.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 4. Selective response of 1(ONPs) (10 mm) to Co2 + over other cations.
when excited at 285 nm, 1(ONPs) exhibited a strong emission at 580 nm with a high quantum yield (F1 = 0.111) using quinine bisulfate in 1 m H2SO4 (F = 0.520) as the reference. As shown in Figures S2 and 4, the addition of 10 equiv of Hg2 + , Pb2 + , Mn2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , Fe2 + , Fe3 + and Cd2 + had no obvious influence on the fluorescence emission. When 10 equiv of Co2 + ions were added to a solution of 1(ONPs) (10 mm), a complete fluorescent blue shift with a small enhancement was observed (F2 = 0.124) from 580 to 485 nm in the maximum emission, which suggested that 1(ONPs) showed a particular response to the Co2 + ions. The observed phenomena are explained on the basis of metal-toligand charge transfer (MLCT) and coordination to a paramagnetic Co2 + center.[18] The fluorescence spectra of the receptor were recorded in DMSO. A binding study of the receptor with Co2 + was also carried out in DMSO. The fluorescence intensity of the receptor in organic solvents, such as DMSO or dimethylformamide (DMF), was low (see Figure S6). A small enhancement (only 2.5-fold) was observed in the presence of Co2 + ions (Figure S7). whereas in the case of the ONPs, the enhancement was very large (7.5-fold). Competitive experiments were carried out with the aim of interference evaluation in the presence of other metal ions for the detection of Co2 + . 1(ONPs) was treated with 10 equiv of Co2 + in the presence of 20 equiv of all other metal ions. The results indicate that the miscellaneous competitive cations did not lead to any significant spectral change and the Co2 + ions still resulted in similar spectral changes in the presence of competitive cations (Figure 5). The data clearly suggests that there is no interference of other metal ions for the sensing of Co2 + . Using the fluorescence titration data, the detection limit of 1(ONPs) for Co2 + was obtained by plotting a graph between the relative emission intensity at 490 nm as a function of the Co2 + concentrations. The emission intensity of receptor 1 was linearly proportional to the Co2 + concentrations (Figure 6 inset). The detection limit based on the IUPAC definition (CDL = 3 Sb m 1) was found to be 88 nm from ten blank solutions (Figure S3). The nanomolar detection limit of receptor 1 was found to be much lower than those of other recently reported Co2 + -selective sensors with similar recognition units. The linear correlation of F490/F435 versus the concentration of Co + 2 ions is plotted in Figure S8. Titration showed a good linearity within the Co + 2 concentration range of 20–80 mm. ChemPhysChem 2014, 15, 3933 – 3937
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Figure 5. Competitive binding assay with ONPs of 1 (10 mm) for Co2 + (10 equvi.) in the presence of other metal ions in an aqueous medium.
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Figure 7. HOMO and LUMO in receptor 1.
HOMO–LUMO gap decreases as a result of the increase in conjugation through the Co2 + ion. As the HOMO–LUMO gap decreases, the wavelength of the absorbed color increases, which authenticates the results obtained by UV/Vis spectroscopy. The lone pair of the Schiff-base nitrogen causes a photoinduced electron transfer (PET) in the receptor molecule. On addition of cobalt ions, the nitrogen donates its excess baggage of electrons to cobalt causing reduced PET. Hence, an extensive enhancement in the photophysical behavior of the receptor–Co complex is observed. The above phenomenon can be perceived from Figure 8, which shows that the 2+ Figure 6. Changes in the fluorescence spectrum of 1(ONPs) (10 mm) upon successive addition of Co . The inset HOMO that was concentrated on shows the linear relationship between fluorescence intensity and [Co2 + ]. 2-hydroxy-3-isopropyl-6-methylbenzaldehyde in receptor 1 has been shifted to the thiophenol ring in the receptor–Co2 + complex and the LUMO is distributed on DFT Calculations for Receptor 1 the disulfide linkage. DFT calculations on receptor 1 were performed using the GGA-DFT package of DMol3[19, 20] to look for potential binding or interacting sites for the incoming cation moieties. The hydroxyl group of receptor 1 interacts with the Schiff-base nitrogen through a hydrogen bond, which leads to keto–enol tautomerism. It was observed that the highest occupied molecular orbital (HOMO) was concentrated around disulfide linkage and 2-hydroxy-3-isopropyl-6-methylbenzaldehyde in the receptor, while the lowest unoccupied molecular orbital (LUMO) was spread on the thiophenol ring of the receptor (Figure 7). DFT studies were extended further to validate the changes in the photophysical response of receptor 1, and modulations were observed on addition of Co2 + ions. On addition of the cobalt ions, the HOMO and LUMO of the receptor–Co2 + complex becomes very stable, as compared to their receptor analogues. Also the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Conclusions A fluorescent dipodal material bearing imine linkages and hydroxyl groups has been synthesized and successfully processed into organic ONPs with an average particle size of 79 nm. The 1(ONPs) were investigated for their recognition properties towards metal ions/anions, and the system was found to be highly selective and sensitive to Co2 + , with a detection limit of 88 nm in water. The lower detection limits possible applications in physiological and environmental systems.
Keywords: cobalt · colorimetry · fluorescence · organic nanoparticles · sensors
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[7] a) A. Singh, T. Raj, T. Aree, N. Singh, Inorg. Chem. 2013, 52, 13830 – 1383; b) A. Singh, S. Sinha, R. Kaur, N. Kaur, N. Singh, Sens. Actuators B 2014, 204, 617 – 621. [8] U. Fegade, S. K. Sahoo, A. Singh, P. Mahulikar, S. Attarde, N. Singh, A. Kuwar, RSC Adv. 2014, 4, 15288 – 15292. [9] a) O. Karovic, I. Tonazzini, N. Rebola, E. Edstrçm, C. Lçvdahl, B. B. Fredholm, Dare, Biochem. Pharmacol. 2007, 73, 694; b) R. Lauwerys, D. Lison, Sci. Total Environ. 1994, 150, 1. [10] R. Suardi, L. Belotti, M. T. Ferrari, P. Leghissa, M. Caironi, L. Maggi, F. Alborghetti, T. Storto, T. Silva, S. Piazzolla, Sci. Total Environ. 1994, 150, 197. [11] K. Redda, L. A. Corleto, E. E. Knaus, J. Med. Chem. 1979, 22, 1079. [12] D. R. Williams, The metal of Life the solution chemistry of metal ions in biological system, Van Nostrand, London, 1971. [13] M. J. Seven, L. A. Johnson, Metal Figure 8. Comparison of the HOMO and LUMO of receptor 1 with those of the receptor1–Co2 + complex in water. Binding in Medicine, 4th Ed., Lippincott Co, Philadelphia P.A, 1960. [14] H. G. Seiler, A. Siegel, H. Siegel, Handbook on Metals in Clinical Analytical Chemistry, Marcel Dekker, New York, 1994, pp. 333 – 338. [1] a) A. Jana, K. Sanjana, P. Devi, T. K. Maiti, N. D. S. Pradeep, J. Am. Chem. [15] a) H. Kasai, H. S. Nalwa, H. Oikawa, S. Okada, H. Matsuda, N. Minami, A. Soc. 2012, 134, 7656 – 7659; b) B. K. An, S. K. Kwon, S. Park, Angew. Kakuta, K. Ono, A. Mukoh, H. Nakanishi, Jpn. J. Appl. Phys. 1992, 31, Chem. Int. Ed. 2007, 46, 1978 – 1982; Angew. Chem. 2007, 119, 2024 – L1132 – L1134; b) H. Kasai, H. Kamatani, S. Okada, H. Oikawa, H. Matsuda, 2028; c) X. Xu, J. Li, Q. Li, J. Huang, Y. Dong, Y. Hong, J. Yan, J. Qin, Z. Li, H. Nakanishi, Jpn. J. Appl. Phys. 1996, 35, L221 – L223. B. Z. Tang, Chem. Eur. J. 2012, 18, 7278 – 7286. [16] J. Suk, A. J. Bard, J. Solid State Electrochem. 2011, 15, 2279 – 2291. [2] a) M.-J. Li, Z. Chen, V. W.-W. Yam, Y. Zu, ACS Nano 2008, 2, 905 – 912; [17] a) T. Raj, P. Saluja, N. Singh, D. O. Jang, RSC Adv. 2014, 4, 5316 – 5321; b) Y.-L. Chang, R. E. Palacios, F.-R. Fan, A. J. Bard, P. F. Barbara, J. Am. b) A. Singh, S. Kaur, N. Singh, N. Kaur, Org. Biomol. Chem. 2014, 12, Chem. Soc. 2008, 130, 8906 – 8907; c) Wang, L. Z. Fan, Z. Wang, H. Liu, Y. 2302 – 2309; c) V. K. Bhardwaj, H. Sharma, N. Kaur, N. Singh, New J. Li, S. Yang, Adv. Mater. 2007, 19, 3677. Chem. 2013, 37, 4192 – 4198. [3] a) L. Pan, Q. He, J. Liu, Y. Chen, M. Ma, L. Zhang, J. Shi, J. Am. Chem. Soc. [18] S. Patil, U. Fegade, S. K. Sahoo, A. Singh, J. Marek, N. Singh, R. Bendre, 2012, 134, 5722 – 5725; b) J. Cheng, K. Wei, X. Ma, X. Zhou, H. Xiang, J. A. Kuwar, ChemPhysChem 2014, 15, 2230 – 2235. Phys. Chem. C 2013, 117, 16552 – 16563; c) J. Cheng, K. Wei, X. Ma, X. [19] C. Lee, W. Yang, R. G. Parr, Phys. Rev.1988, B37, 785. Zhou, H. Xiang, Chem. Commun. 2013, 49, 11791; d) J. Cheng, X. Ma, Y. [20] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270. Zhang, J. Liu, X. Zhou, H. Xiang, Inorg. Chem. 2014, 53, 3210 – 3219. [4] L. Li, K. Liu, G. Yang, C. Wang, J. Zhang, J. Zhu, Adv. Funct. Mater. 2011, 21, 869. Received: July 22, 2014 [5] L. Xi, H. Fu, W. Yang, J. Yao, Chem. Commun. 2005, 492 – 494. Revised: September 19, 2014 [6] A. Jana, B. Saha, D. R. Banerjee, S. K. Ghosh, K. T. Nguyen, X. Ma, Q. Qu, Published online on October 15, 2014 Y. Zhao, N. D. Pradeep Singh, Bioconjugate Chem. 2013, 24, 1828 – 1839.
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A Fluorescent and Colorimetric Sensor for Nanomolar Detection of Co2 + in Water Anil Kuwar,*[a] Rahul Patil,[a] Amanpreet Singh,[b] Ratnamala Bendre,[a] and Narinder Singh*[b] A new disulfide-based, imine-linked fluorescent receptor 1 was processed into organic nanoparticles (ONPs) with an average particle size of 79 nm. The photophysical properties of the ONPs were evaluated by UV/Vis absorption spectroscopy. Receptor 1 selectively recognized Co2 + ions in water with a detection limit down to 88 nm.
Organic nanoparticles (ONPs) prepared from a variety of materials, including natural products, polymers and biomolecules, have found exciting applications in materials and biological sciences.[1–5] With respect to inorganic nanoparticles, ONPs are anticipated to allow much more structural changeability through well-known synthetic strategies. Moreover, ONPs dispersed in water validated the utilize of probes in drug delivery system (DDS) and cellular imaging.[6] Singh et al. have reported systematic work on ONPs with the use of Biginelli-Based and other Schiff base nanoparticles that disclosed the size-dependent fluorescent properties of organic materials.[7] For biological and chemical analysis, the probe should be soluble in pure water; however most of the chemosensors are hydrophobic so freshly an innovative concept has been created, which include with the synthesis of ONPs.[8] In sensor development, the ratiometric fluorescence determination of the analyte has great advantages over the conventional monitoring of fluorescence intensity at a single wavelength. The detection of Co2 + among the transition-metal ions has attracted much attention because cobalt is one of the most important trace elements in human beings. In the structure of vitamin B12 (cobalamin), this metal takes part in a number of critical tasks in many biological purposes.[9] Cobalamin is essential for DNA synthesis, the formation of red blood cells, the continuation of the nervous system, and the growth and expansion of children.[10] Similar to other critical elements, cobalt can be less toxic than non-vital metals like platinum.[11] However, because of the crucial biological role of cobalt throughout the metabolism of mammals, its imbalance may lead to diseases and malignancies.[12] The excess of cobalt from the normal permissible limit can cause detrimental effects such as vasodi[a] Dr. A. Kuwar, R. Patil, Prof. R. Bendre School of Chemical Sciences, North Maharashtra University Jalgaon—425001 MS (India) E-mail: [email protected] [email protected] [b] A. Singh, Dr. N. Singh Department of Chemistry, Indian Institute Technology Roapr-140001 Punjab (India) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402534.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
latation, flushing and cardiomyopathy, whereas its deficiency in humans and animals results in anemia.[13] Because of the potential need for a highly sensitive analytical method, there is immense interest in the development of Co2 + -selective chemosensors for various biological and environmental applications.[14] Our current research work is focused on the synthesis of fluorescence-based organic nanoparticles for the sensitive and selective detection of particular cations over other cations in water. In this assembly, sulfur, imine, and hydroxyl groups are bearing responsible binding sites for cation recognition. Herein, we explored disulfide–imine-based ONPs (1) as a highly selective “turn-on” fluorescence sensor for Co2 + ions.
Experimental Section All spectroscopy-grade chemicals and solvents were purchased from Aldrich and used without further purification. Fluorescence and UV/Vis spectra were recorded on a Fluoromax-4 Spectrofluorometer and a Shimadzu UV-24500 equipment, respectively, with a 5 nm slit of fluorescence; the nitrate salts of the metal were used in the study. Ultrapure water obtained with a Millipore Purification System (Milli-Q water) was used throughout the analytical experiments. 1H NMR spectra were recorded on a Jeol spectrometer operating at 400 MHz in [D6]DMSO.
Synthesis of Receptor 1 Receptor 1 was synthesized by reacting one mole of 2-(2-(2-aminophenyl)disulfanyl)benzenamine (0.24 g, 1 mm) with one mole of 2hydroxy-3-isopropyl-6-methylbenzaldehyde (0.17 g, 1 mm) in ethanol, with stirring and refluxing for three hours at ambient temperature. Receptor 1 was obtained in good yield as a yellow powder. Yield: 86 %, m.p. 156 8C. 1H NMR (400 MHz, [D6]DMSO, ppm) d = 1.22 (d, 12 H, gem 4 CH3), 2.25 (s, 6 H, Ar-CH3), 2.85 (septet, 2 H, CH), 6.98(d, 2 H, Ar-H), 7.06(d, 2 H, Ar-H), 7.10–7.32 (m, 8 H, Ar-H), 8.55 (s, 2 H, -CH = N), 11.09 (s, 2 H, -OH). 13C NMR (100 MHz, [D6]DMSO, ppm) d = 15.93, 24.12, 29.08, 113.89, 114.87, 114.98, 124.97, 126.53, 127, 130.91, 131.26, 141.16, 142.79, 143.93, 156.23, 166.36; CHN analysis: Calculated = C 71.79, H 6.38, N 4.93. Obtained = C 71.64, H 6.18, N 4.69.
Synthesis of the Organic Nanoparticles (ONPs) The ONPs were obtained by the reprecipitation method,[15] in which a solution of the organic compound is injected into water and the resulting organic nanoparticles are obtained as a result of precipitation. This method is simple in operation and the particles obtained are usually about ten to hundreds of nanometers in diameter. Briefly, for the preparation of the ONPs, 5 mg of receptor 1 was first dissolved in a minimum amount of dimethyl sulfoxide (DMSO, 1 mL). This receptor solution was taken in a syringe and inChemPhysChem 2014, 15, 3933 – 3937
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of the organic compound is injected into water and the resulting organic nanoparticles are obtained as a result of precipitation. This involves the injection of the organic compound [1 mL of a stock solution of 1(ONPs) in pure DMSO] to 100 mL of distilled water. There is great disparity between the solubility of 1(ONPs) in DMSO (good solubility) and water (highly insoluble); however, the good solubility/ miscibility compatibility of the two solvents is the governing feature of the method.[16] Sonication ensured the rapid mixing of both the solutions. Receptor 1 is exposed to water for a very short time, and the water is expected to change the microenvironment of 1(ONPs) inducing the nucleation and growth of molecules to nanoparticles. The particle-size distribution of the nano-particles was estimatFigure 1. a) Particle-size analysis of 1(ONPs), measured by DLS. b) TEM analysis showing the particle size of 1(ONPs). c) Comparison of the UV/Vis absorption spectra of ligand 1 (red line) and 1(ONPs) (blue line). ed by DLS; the studies revealed that the size of the 1(ONPs) was in the range from 50 to 120 nm, approximately, whereas the average size of the 1(ONPs) was found to jected to 100 mL of distilled water in small increments under sonibe 79 nm (Figure 1 a). The spherical shape of the 1(ONPs) was cation, which was continued for about half an hour to ensure the demonstrated by TEM analysis, as ashown in Figure 1 b. An assesspreparation of stabilized ONPs; these materials are denoted as ment of the UV/Vis absorption spectra of 1 (recorded in pure 1(ONPs) throughout the manuscript. During the course of solution DMSO) with 1(ONPs) (recorded in water) revealed changes in the mixing, the size of the nanoaggregates was analyzed by DLS (difphotophysical properties on aggregation of the material.[17] From ferential light scattering) and was found to be 79 nm, Moreover, the comparison of the UV/Vis absorption spectra of receptor 1 and TEM (transmission electron microscopy) images were taken to 1(ONPs), it is concluded that the excitation band for receptor check the uniformity of the nanoaggregates (Figure 1 b). The TEM 1 occurs at 345 nm, whereas for 1(ONPs) the band occurs at analysis revealed that the particles are uniformly distributed and 285 nm, and moreover, the band for receptor 1 broadens after the have sizes smaller than 70 nanometer. formation of 1(ONPs) (Figure 1 c). Initially, the colorimetric detection ability of receptor 1(ONPs) was studied by treating it with variUV/Vis and Fluorescence Spectral Measurements ous cations (2 equivalents). It was observed that the color of the receptor changes from colorless to brown only in the case of Co2 + The metal ions Hg2 + , Pb2 + , Mn2 + , Co2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , salts. The color change could be detected by the naked eye Fe2 + , Fe3 + and Cd2 + were added as their nitrate salts for absorp(Figure 2 inset). However, the addition of other cations did not tion and fluorescence spectroscopic experiments. Stock solutions result in detectable color changes. 3 4 of the metal ions (1 10 m) and the receptor 1(ONPs) (1 10 m) were prepared in water. These stock solutions were used after apThe changes in the UV/Vis spectra of 1(ONPs) (10 mm), caused by propriate dilution. Co2 + (10 equiv) and miscellaneous cations (10 equiv) including
Results and Discussion Receptor 1 was synthesized by a condensation reaction between 2-(2-(2-aminophenyl)disulfanyl)benzenamine and one mole of 2-hydroxy-3-isopropyl-6-methylbenzaldehyde in ethanol (Scheme 1). The condensation product 1 was characterized by using spectroscopic methods, that is, the formation of CH = N- bond was established from NMR; while elemental analysis (CHN) gave the correct structural formula of the product. ONPs of receptor 1(ONPs) were obtained by the the re-precipitation method,[15] in which a solution Scheme 1. Synthesis of receptor 1. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
alkali, alkaline earth, and transition-metal ions, Hg2 + , Pb2 + , Mn2 + , Co2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , Fe2 + , Fe3 + and Cd2 + in water solution, are recorded in Figure 2. The various alkali, alkaline earth, and transition metal ions did not induce any apparent absorption changes, even upon addition of 10 equiv of the respective metal ions. Addi-
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Figure 2. UV/Vis spectra of 1(ONPs) (10 mm) in water in the presence 10 equiv of Co2 + ions and other cations including Hg2 + , Pb2 + , Mn2 + , Co2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , Fe2 + , Fe3 + , and Cd2 + . The inset shows the visual change in the solution of receptor 1(ONPs) upon addition of Co2 + . Left: receptor 1(ONPs); right: receptor 1(ONPs) + Co2 + .
tion of 10 equiv of Co2 + to a solution of 1(ONPs) immediately resulted in a significant red-shift of the absorbance from 357 to 457 nm, as well as a color change of the solution from colorless to brown. These facts suggested that the 1(ONPs) behaved as an efficient colorimetric probe for the detection of Co2 + ions owing to the fact that the color changes can be easily distinguished by the eye (Figure 2 inset). In the UV/Vis titration of Co2 + ions, the 1(ONPs) showed characteristic absorption peaks at 357 and 457 nm. Upon treatment with Co2 + ions, the absorption peak at 357 nm decreased whereas that at 457 nm increased, highlighting the occurrence of an isosbestic point at 395 nm (Figure 3). The anion recognition properties of 1(ONPs) have also been investigated by monitoring the change in the fluorescence spectra upon the addition of different tetrabutylammonium salts of different anions to the host solution. A solution of 1(ONPs) (10 mm) was mixed with aliquots of different anions (20 equivalent) such as F , Cl , Br , I , CH3COO , NO3 , H2PO4 , HSO4 and ClO4 . All photoluminescence measurements were performed under the same conditions as those used to study cation recognition. No significant changes in the fluorescence spectra were observed upon addition of any of the tested anions (Figure S1 of the Supporting Information, SI). To obtain further insights into the Co2 + sensing properties of 1(ONPs), the emission was examined with various metal species
Figure 3. Spectra taken during the titration of 1 (ONPs) (10 mm) in water with a standard solution of Co2 + at room temperature.
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Figure 4. Selective response of 1(ONPs) (10 mm) to Co2 + over other cations.
when excited at 285 nm, 1(ONPs) exhibited a strong emission at 580 nm with a high quantum yield (F1 = 0.111) using quinine bisulfate in 1 m H2SO4 (F = 0.520) as the reference. As shown in Figures S2 and 4, the addition of 10 equiv of Hg2 + , Pb2 + , Mn2 + , Ni2 + , Al3 + , Cu2 + , Zn2 + , Fe2 + , Fe3 + and Cd2 + had no obvious influence on the fluorescence emission. When 10 equiv of Co2 + ions were added to a solution of 1(ONPs) (10 mm), a complete fluorescent blue shift with a small enhancement was observed (F2 = 0.124) from 580 to 485 nm in the maximum emission, which suggested that 1(ONPs) showed a particular response to the Co2 + ions. The observed phenomena are explained on the basis of metal-toligand charge transfer (MLCT) and coordination to a paramagnetic Co2 + center.[18] The fluorescence spectra of the receptor were recorded in DMSO. A binding study of the receptor with Co2 + was also carried out in DMSO. The fluorescence intensity of the receptor in organic solvents, such as DMSO or dimethylformamide (DMF), was low (see Figure S6). A small enhancement (only 2.5-fold) was observed in the presence of Co2 + ions (Figure S7). whereas in the case of the ONPs, the enhancement was very large (7.5-fold). Competitive experiments were carried out with the aim of interference evaluation in the presence of other metal ions for the detection of Co2 + . 1(ONPs) was treated with 10 equiv of Co2 + in the presence of 20 equiv of all other metal ions. The results indicate that the miscellaneous competitive cations did not lead to any significant spectral change and the Co2 + ions still resulted in similar spectral changes in the presence of competitive cations (Figure 5). The data clearly suggests that there is no interference of other metal ions for the sensing of Co2 + . Using the fluorescence titration data, the detection limit of 1(ONPs) for Co2 + was obtained by plotting a graph between the relative emission intensity at 490 nm as a function of the Co2 + concentrations. The emission intensity of receptor 1 was linearly proportional to the Co2 + concentrations (Figure 6 inset). The detection limit based on the IUPAC definition (CDL = 3 Sb m 1) was found to be 88 nm from ten blank solutions (Figure S3). The nanomolar detection limit of receptor 1 was found to be much lower than those of other recently reported Co2 + -selective sensors with similar recognition units. The linear correlation of F490/F435 versus the concentration of Co + 2 ions is plotted in Figure S8. Titration showed a good linearity within the Co + 2 concentration range of 20–80 mm. ChemPhysChem 2014, 15, 3933 – 3937
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Figure 5. Competitive binding assay with ONPs of 1 (10 mm) for Co2 + (10 equvi.) in the presence of other metal ions in an aqueous medium.
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Figure 7. HOMO and LUMO in receptor 1.
HOMO–LUMO gap decreases as a result of the increase in conjugation through the Co2 + ion. As the HOMO–LUMO gap decreases, the wavelength of the absorbed color increases, which authenticates the results obtained by UV/Vis spectroscopy. The lone pair of the Schiff-base nitrogen causes a photoinduced electron transfer (PET) in the receptor molecule. On addition of cobalt ions, the nitrogen donates its excess baggage of electrons to cobalt causing reduced PET. Hence, an extensive enhancement in the photophysical behavior of the receptor–Co complex is observed. The above phenomenon can be perceived from Figure 8, which shows that the 2+ Figure 6. Changes in the fluorescence spectrum of 1(ONPs) (10 mm) upon successive addition of Co . The inset HOMO that was concentrated on shows the linear relationship between fluorescence intensity and [Co2 + ]. 2-hydroxy-3-isopropyl-6-methylbenzaldehyde in receptor 1 has been shifted to the thiophenol ring in the receptor–Co2 + complex and the LUMO is distributed on DFT Calculations for Receptor 1 the disulfide linkage. DFT calculations on receptor 1 were performed using the GGA-DFT package of DMol3[19, 20] to look for potential binding or interacting sites for the incoming cation moieties. The hydroxyl group of receptor 1 interacts with the Schiff-base nitrogen through a hydrogen bond, which leads to keto–enol tautomerism. It was observed that the highest occupied molecular orbital (HOMO) was concentrated around disulfide linkage and 2-hydroxy-3-isopropyl-6-methylbenzaldehyde in the receptor, while the lowest unoccupied molecular orbital (LUMO) was spread on the thiophenol ring of the receptor (Figure 7). DFT studies were extended further to validate the changes in the photophysical response of receptor 1, and modulations were observed on addition of Co2 + ions. On addition of the cobalt ions, the HOMO and LUMO of the receptor–Co2 + complex becomes very stable, as compared to their receptor analogues. Also the 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Conclusions A fluorescent dipodal material bearing imine linkages and hydroxyl groups has been synthesized and successfully processed into organic ONPs with an average particle size of 79 nm. The 1(ONPs) were investigated for their recognition properties towards metal ions/anions, and the system was found to be highly selective and sensitive to Co2 + , with a detection limit of 88 nm in water. The lower detection limits possible applications in physiological and environmental systems.
Keywords: cobalt · colorimetry · fluorescence · organic nanoparticles · sensors
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[7] a) A. Singh, T. Raj, T. Aree, N. Singh, Inorg. Chem. 2013, 52, 13830 – 1383; b) A. Singh, S. Sinha, R. Kaur, N. Kaur, N. Singh, Sens. Actuators B 2014, 204, 617 – 621. [8] U. Fegade, S. K. Sahoo, A. Singh, P. Mahulikar, S. Attarde, N. Singh, A. Kuwar, RSC Adv. 2014, 4, 15288 – 15292. [9] a) O. Karovic, I. Tonazzini, N. Rebola, E. Edstrçm, C. Lçvdahl, B. B. Fredholm, Dare, Biochem. Pharmacol. 2007, 73, 694; b) R. Lauwerys, D. Lison, Sci. Total Environ. 1994, 150, 1. [10] R. Suardi, L. Belotti, M. T. Ferrari, P. Leghissa, M. Caironi, L. Maggi, F. Alborghetti, T. Storto, T. Silva, S. Piazzolla, Sci. Total Environ. 1994, 150, 197. [11] K. Redda, L. A. Corleto, E. E. Knaus, J. Med. Chem. 1979, 22, 1079. [12] D. R. Williams, The metal of Life the solution chemistry of metal ions in biological system, Van Nostrand, London, 1971. [13] M. J. Seven, L. A. Johnson, Metal Figure 8. Comparison of the HOMO and LUMO of receptor 1 with those of the receptor1–Co2 + complex in water. Binding in Medicine, 4th Ed., Lippincott Co, Philadelphia P.A, 1960. [14] H. G. Seiler, A. Siegel, H. Siegel, Handbook on Metals in Clinical Analytical Chemistry, Marcel Dekker, New York, 1994, pp. 333 – 338. [1] a) A. Jana, K. Sanjana, P. Devi, T. K. Maiti, N. D. S. Pradeep, J. Am. Chem. [15] a) H. Kasai, H. S. Nalwa, H. Oikawa, S. Okada, H. Matsuda, N. Minami, A. Soc. 2012, 134, 7656 – 7659; b) B. K. An, S. K. Kwon, S. Park, Angew. Kakuta, K. Ono, A. Mukoh, H. Nakanishi, Jpn. J. Appl. Phys. 1992, 31, Chem. Int. Ed. 2007, 46, 1978 – 1982; Angew. Chem. 2007, 119, 2024 – L1132 – L1134; b) H. Kasai, H. Kamatani, S. Okada, H. Oikawa, H. Matsuda, 2028; c) X. Xu, J. Li, Q. Li, J. Huang, Y. Dong, Y. Hong, J. Yan, J. Qin, Z. Li, H. Nakanishi, Jpn. J. Appl. Phys. 1996, 35, L221 – L223. B. Z. Tang, Chem. Eur. J. 2012, 18, 7278 – 7286. [16] J. Suk, A. J. Bard, J. Solid State Electrochem. 2011, 15, 2279 – 2291. [2] a) M.-J. Li, Z. Chen, V. W.-W. Yam, Y. Zu, ACS Nano 2008, 2, 905 – 912; [17] a) T. Raj, P. Saluja, N. Singh, D. O. Jang, RSC Adv. 2014, 4, 5316 – 5321; b) Y.-L. Chang, R. E. Palacios, F.-R. Fan, A. J. Bard, P. F. Barbara, J. Am. b) A. Singh, S. Kaur, N. Singh, N. Kaur, Org. Biomol. Chem. 2014, 12, Chem. Soc. 2008, 130, 8906 – 8907; c) Wang, L. Z. Fan, Z. Wang, H. Liu, Y. 2302 – 2309; c) V. K. Bhardwaj, H. Sharma, N. Kaur, N. Singh, New J. Li, S. Yang, Adv. Mater. 2007, 19, 3677. Chem. 2013, 37, 4192 – 4198. [3] a) L. Pan, Q. He, J. Liu, Y. Chen, M. Ma, L. Zhang, J. Shi, J. Am. Chem. Soc. [18] S. Patil, U. Fegade, S. K. Sahoo, A. Singh, J. Marek, N. Singh, R. Bendre, 2012, 134, 5722 – 5725; b) J. Cheng, K. Wei, X. Ma, X. Zhou, H. Xiang, J. A. Kuwar, ChemPhysChem 2014, 15, 2230 – 2235. Phys. Chem. C 2013, 117, 16552 – 16563; c) J. Cheng, K. Wei, X. Ma, X. [19] C. Lee, W. Yang, R. G. Parr, Phys. Rev.1988, B37, 785. Zhou, H. Xiang, Chem. Commun. 2013, 49, 11791; d) J. Cheng, X. Ma, Y. [20] P. J. Hay, W. R. Wadt, J. Chem. Phys. 1985, 82, 270. Zhang, J. Liu, X. Zhou, H. Xiang, Inorg. Chem. 2014, 53, 3210 – 3219. [4] L. Li, K. Liu, G. Yang, C. Wang, J. Zhang, J. Zhu, Adv. Funct. Mater. 2011, 21, 869. Received: July 22, 2014 [5] L. Xi, H. Fu, W. Yang, J. Yao, Chem. Commun. 2005, 492 – 494. Revised: September 19, 2014 [6] A. Jana, B. Saha, D. R. Banerjee, S. K. Ghosh, K. T. Nguyen, X. Ma, Q. Qu, Published online on October 15, 2014 Y. Zhao, N. D. Pradeep Singh, Bioconjugate Chem. 2013, 24, 1828 – 1839.
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