Selective Dopamine Chemosensing Using Silver-Enhanced

Mar 20, 2014 - solution exhibits intense silver nanoparticle enhanced fluorescence (SEF) with the λem at 412 nm. Dopamine is selectively detected dow...
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Selective Dopamine Chemosensing Using Silver-Enhanced Fluorescence Mainak Ganguly,† Chanchal Mondal,† Jayasmita Jana,† Anjali Pal,‡ and Tarasankar Pal*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India

‡

S Supporting Information *

ABSTRACT: Condensation product of salicylaldehyde and 1,3 propylenediamine becomes a diiminic Schiff base, which is oxidized by AgNO3 in alkaline solution, and in turn, stable Ag(0) is produced at room temperature. Under this condition, the solution exhibits intense silver nanoparticle enhanced fluorescence (SEF) with the λem at 412 nm. Dopamine is selectively detected down to the nanomolar level via exclusive fluorescence quenching of the SEF. Dopamine-infested solution regains the fluorescence [i.e., SEF in the presence of Hg(II) ions]. Thus dopamine and Hg(II) in succession demonstrate “turn off/on” fluorescence due to the change in the scattering cross section of Ag(0) and gives a quantitative measure of dopamine in real samples. The proposed method is free from interferences of common biocompetitors.

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INTRODUCTION

Metal nanoparticles (MNPs) have attracted enormous attention in the arena of fluorescence spectroscopy serving as nanoantennas. Quite often, they manipulate light and light− matter interactions at the nanoscale. The absorption capability and radiative decay rate of emitters placed in their near field14−16 are influenced at ease by the localized plasmon polaritons of MNPs. Mohammadi et al. reported nanoantennas of spheroidal MNPs as a function of aspect ratio, background index, volume, and type of metal.15 Many times, gold and silver MNPs have served as nanoantennas.17−21 Because of the cost of generating gold surfaces, they are not a good choice for appliances involving metal-enhanced fluorescence (MEF). The cheaper silver, with a favorable imaginary component of dielectric constant, contributes a new dimension in this context. Here, a diiminic Schiff base has been used for the synthesis of Ag(0) nanoparticles in solution. As a consequence, the in situ produced Ag(0) causes intriguing enhancement of fluorescence of the oxidized Schiff base. Amino acids, other common biocompetitors like ascorbic acid, uric acid, glucose, lactose, and Na+, K+, and Zn2+ ions do not interfere in the context of unique

In the mammalian central nervous system, dopamine (DA) has been known to be a vital catecholamine neurotransmitter.1−4 Deviation of normal DA concentration in the brain is dangerous, causing nervous disorders and Parkinson’s disease.3,4 A major problem in DA determination is poor spectral resolution. The coexisting compounds, mainly ascorbic acid (AA) and uric acid (UA), always interfere making DA determination highly problematic.5 Over the course of a few decades, the electroactive nature of DA has encouraged researchers to adopt electrochemical methods for the determination of DA. Unfortunately, overlapping voltammetric response is observed because of the proximity of the potential for AA, UA, and DA oxidation at the solid electrode surface. To avoid this selectivity problem, several methods with chemically and physically modified electrodes have also been tried.6−9 Besides, high-performance liquid chromatography−mass spectrometry10 as well as capillary electrophoresis with laserinduced native fluorescence11 has been reported. But, such practices are not only complicated, time-consuming, and expensive but also demand specialized equipment. Recently, a technique involving simple visible spectrophotometry12 as well as fluorimetry13 has been reported with decent detection limits. © 2014 American Chemical Society

Received: December 28, 2013 Revised: March 15, 2014 Published: March 20, 2014 4120

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Scheme 1. SEF is Quenched by Oxidized DA (QDA) Replacing QL from the Vicinity of Ag(0)

Figure 1. (A) Powder XRD spectrum of the precipitate obtained from AgQL solution. (B) Fringe spacing of Ag particles present in AgQL solution (obtained from HRTEM image). (C) XPS spectrum of the AgQL solution under the freeze-drying condition for the element silver.

silver-enhanced fluorescence (SEF). Then DA has appeared as a selective fluorescence quencher of the solution bearing SEF. Thus selective and sensitive determination of DA has been possible, employing SEF in solution phase.

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aqua regia, all glassware was cleaned and subsequently rinsed with a copious amount of distilled water. Then, they were dried well before use. With a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito, India), all UV−vis absorption spectra were recorded. FTIR spectra were obtained using a FT-IR Nexus spectrophotometer (Thermo Nicolet). 1H NMR spectra were recorded with a 400 MHz Bruker NMR instrument. X-ray photoelectron spectroscopy (XPS) analysis was performed with the help of 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 done using an LS55 fluorescence spectrometer (Perkin-Elmer). Fluorescence lifetimes were measured with Easy lifeR

EXPERIMENTAL SECTION

Material and Instrument. All the reagents were of AR grade. Triple-distilled water was used throughout the experiment. Slver nitrate (AgNO3), salicylaldehyde, 1,3-propanediamine, L-DOPA, dopamine, ascorbic acid, amino acids, lactose, glucose, zinc acetate, sodium sulfate, and potassium were purchased from Sigma-Aldrich. From HiMedia Laboratories Pvt. Ltd., NaOH was obtained. All the reagents were used without further purification. Using freshly prepared 4121

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(Optical Building Blocks Corporation) equipped with a 295 nm LED excitation source. TEM analysis was carried out with an H-9000 NAR instrument (Hitachi), using an accelerating voltage of 300 kV. Synthesis of L. 1,3-propanediamine (10−2 mol) in methanol was gently added in methanolic salicylaldehyde (2 × 10−2 mol) solution with constant stirring. Then, the mixture was refluxed for ∼4 h. A yellow precipitate gradually appeared after cooling. By filtration and washing 2−3 times with methanol, the yellow product (L) was obtained.19,20 It was recrystallized from methanol [characterized from melting point (47 °C), 1H NMR and IR; Figures S1, S2, and the Supporting Information]. Synthesis of AgQL. Schiff base L was synthesized through a condensation reaction of salicylaldehyde and 1,3-propylenediamine. The bright-yellow solid L was water insoluble and dissolved in 0.1 M NaOH medium. A stock solution of 2.5 × 10−3 M L was prepared by dissolving an appropriate amount of L in a 0.1 M aqueous NaOH solution. An aliquot of 0.2 mL of L solution was mixed with 3.2 mL of 0.1 M aqueous NaOH and 0.1 mL of 10−2 M AgNO3 solution. Immediately, a brown color appeared due to the formation of AgOH. However, the bulk solution appeared yellow due to the presence of intense yellow L present in the solution. Now, the alkaline solution containing AgOH and L was allowed to stand for a day. As a result, AgOH was slowly reduced to Ag(0) and L was oxidized to QL at room temperature. Then the solution was centrifuged at 5000 rpm for 10 min to remove coarse particles from the solution, and the strongly fluorescent clear AgQL solution was obtained (Figure S3 of the Supporting Information). The solution exhibits maximum fluorescence upon standing, while [L]/[Ag(I)] = 0.5 (Figure S4 of the Supporting Information). This indicates the quantitative oxidation of L into QL. So, one molecule of L was required to reduce two Ag(I) ions. A single-emission peak at ∼412 nm in the fluorescence spectrum and unicomponent fluorescence lifetime further support quantitative oxidation of L. Here, L behaves as a reducing agent and the in situ produced QL acts as a capping agent for Ag(0) particles. The plasmon band for AgNPs in the AgQL solution at ∼400 nm remained concealed within the absorption band of QL (Figure S5 of the Supporting Information).

Figure 2. Fluorescence spectral profile after adding different metal ions in alkaline L solution (followed by aging for one day); [L] = 1.4 × 10−4 M and [Mn+] = 2.8 × 10−4 M.

mammoth fluorescence enhancement of QL due to a higher scattering cross-section of silver as reported by Lukomska et al.25 Solution bestowing huge SEF bears a faint yellow color. Now, addition of DA to that solution produces faint red coloration (due to quinone formation, QDA) (Figure S5 of the Supporting Information) with remarkable quenching of fluorescence without any alteration of the emission maximum. Amino acids as well as different other biocompetitors (ascorbic acid, UA, Na+, K+, Zn2+, glucose, and lactose)12 of DA are virtually ineffective in the context of unique SEF disclosed herein (Figure 3). (I0 − I)/I0 is ∼1 in the presence of DA and 550 nm.38,39 Here, QL does not possess sulfur and AgQL solution has a strong fluorescence with emission maxima at 412 nm. Such fluorescence is quite unlikely to have origins in ultrasmall silver clusters. When alkaline L solution is kept undisturbed for one day, a new peak appears at 430 nm, due to the formation of QL in portion. However, in the presence of an appropriate amount of silver ([QL]/[Ag(I)] = 0.5), L is quantitatively converted to 4126

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demonstrate fluorescent silver clusters as important candidates for the detection of cysteine and Hg(II). Selective quenching of fluorescence of silver clusters, due to steric factor guided penetrating ability and short path lengths of analytes, has been employed for detection purposes. In our case, strong silverenhanced fluorescence is quenched due to replacement of QL by QDA from the proximity of Ag(0). QDA also splits the large aggregated Ag(0) particles into smaller particles causing a decrement of the scattering cross section and an increment of the absorption cross section. Diluting the electric field around the fluorophore, such small silver particles can make surface wave lossy and decrease the rate of excitation. Little decrease in the radiative dacay rate also speaks for quenching.14 As a result, addition of DA to highly fluorescent AgQL selectively quenches the fluorescence.

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CONCLUSIONS The ease of tactic to obtain SEF, hitherto unknown, may be a wealth for key applications. We expect that the prescribed approach of DA determination may offer a novel route for developing low cost, simple, and sensitive DA biosensors, which is likely to be extremely helpful in the vast arena of the clinical diagnostic, biosensors, and nanotechnology domain. Again, the selective rebirth of fluorescence by Hg(II) is also promising in order to design a Hg(II) sensor, which has become an important area of research to date.

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ASSOCIATED CONTENT

S Supporting Information *

HNMR, IR, XRD, XPS, UV−vis, fluorescence spectra, and fluorescence decay profile. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the UGC, DST, NST, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance. REFERENCES

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