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Journal of Experimental Nanoscience

ISSN: 1745-8080 (Print) 1745-8099 (Online) Journal homepage: http://www.tandfonline.com/loi/tjen20

Rapid synthesis of highly stable silvernanoparticles and its application for colourimetricsensing of cysteine

Sainath Babu, Michelle O. Claville & Kesete Ghebreyessus

To cite this article: Sainath Babu, Michelle O. Claville & Kesete Ghebreyessus (2015)Rapid synthesis of highly stable silver nanoparticles and its application for colourimetricsensing of cysteine, Journal of Experimental Nanoscience, 10:16, 1242-1255, DOI:10.1080/17458080.2014.994680

To link to this article: http://dx.doi.org/10.1080/17458080.2014.994680

Published online: 03 Jan 2015.

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Rapid synthesis of highly stable silver nanoparticles and its applicationfor colourimetric sensing of cysteine

Sainath Babu#, Michelle O. Claville and Kesete Ghebreyessus*,#

Department of Chemistry, Hampton University, Hampton, VA 23668, USA

(Received 13 August 2014; final version received 26 November 2014)

A modified green approach for the synthesis of stable silver nanoparticles(AgNPs) using tea leaf extract is described. The method involves the reduction ofsilver salt by the polyphenols present in the green tea leaf extract and requires noadditional capping/stabilising agents. Compared to other biogenic methods forthe synthesis of AgNPs, the uniqueness of the approach described here lies in itssimplicity, low-cost, and rapid synthesis rate; the reaction being completed within10�15 min at room temperature. The reaction was carried out in alkaline mediumwithout stirring and heating, and requires no special cleaning or drying of theglassware used. The synthesised AgNPs were characterised by UV�Visspectroscopy and transmission electron microscopy (TEM). The results showedthat AgNPs with a strong surface plasmon resonance peak around 410 nm andparticle size in the 5�30 nm range were prepared. The synthesised AgNPs showexcellent chemical stability for more than six months in aqueous solution.Additionally, we showed that the as-synthesised AgNPs can be used as highlyselective colorimetric and optical sensors for the detection of cysteine. Thus, witha simple synthesis strategy, and enhanced stability, these green-tea-functionalisedAgNPs have the potential for further applications as biosensors andantimicrobial agents.

Keywords: colorimetric sensors; metal nanoparticles; naked eye; amino acids;green methods

1. Introduction

Noble metal nanoparticles of gold (AuNPs) and silver (AgNPS) have attracted

considerable attention because of their characteristic surface plasmon resonance that

occurs in the visible region, and their potential applications in several areas ranging

from colorimetric sensors, catalysis, bioimaging, environmental remediation, and cancer

therapy.[1�13] Among the nanomaterials, AgNPs are the most highly commercialised and

extensively studied nanoparticles due to their unique antibacterial and optoelectronic

properties. They are widely used in numerous consumer products, such as medical devices,cleaning agents, personal care products, and textiles.[14�17] The conventional method for

the synthesis of AgNPs involves the reduction of silver ions by reducing agents such as

sodium citrate or sodium borohydride, followed by the addition of capping agents such as

alkanethiols and alkylamines, or polymeric materials such as polyvinyl alcohol, and

*Corresponding author. Email: kesete.ghebreyessus@hamptonu.edu#Both authors contributed equally to this work.

� 2014 Taylor & Francis

Journal of Experimental Nanoscience, 2015

Vol. 10, No. 16, 1242�1255, http://dx.doi.org/10.1080/17458080.2014.994680

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gelatin to stabilise the AgNPs from aggregation.[17�20] However, the use of cappingagents for stabilisation and toxic chemicals for reductions may result in the formation of

hazardous by-products, adversely affecting their use in the biomedical fields and chemical

applications.

In order to overcome these drawbacks, the application of green methods to the

synthesis of nanoparticles is rapidly expanding as the broader utility of such approaches is

realised.[10,11,14,15,17,21�33] A recent review by Varma et al. [21] has highlighted the

applicability of green synthetic approaches using plant extracts, enzymes, micro-

organisms, biodegradable polymers, and microwaves to prepare AgNPs through redoxreactions. While many themes and variations have been developed, the use of plant-based

extracts as both reducing and capping agents has emerged as a simple and attractive

method for the synthesis of AgNPs. This new technique for the green synthesis of AgNPs

offers the distinct advantages of being environmentally friendly, low- cost, and able to

generate nanoparticles with good biocompatibility. To date, many plant parts including

leaves, seeds, stems, roots, and fruits have been successfully used for the synthesis of

nanoparticles.[17,21�35] However, in most cases, longer reaction times (2�5 h) and

constant stirring and heating requirements present some major drawbacks in thoseapproaches.[12,25,26] To our knowledge, the only successful method that describes a fast

synthesis rate of AgNPs, with a reaction time of 15 min or less, was recently reported by

Wang et al.,[32] and involves the use of pear fruit juice as a reducing and capping agent.

Hence, there is a sustained need for the development of simple and rapid synthetic

methods for nanoparticles using eco-friendly natural products based on the principles of

green chemistry.

Herein, we report the facile green synthesis of AgNPs using green tea leaf extracts as

reducing and stabilising agents under alkaline conditions. Compared with the previouslyreported approaches, the method described here is straightforward, simple, reproducible,

cost effective, and rapid. The synthesis was completed within 10�15 min, producing

brown-coloured colloidal solutions of AgNPs at room temperature. The synthesised

AgNPs are highly stable, staying without any noticeable change for more than six months

while kept at room temperature and without any protection from light exposure. We have

also explored the potential applications of the synthesised AgNPs as colourimetric and

optical probes for the detection of cysteine in aqueous solution.

Cysteine was chosen as a detecting target, because sulphur-containing amino acids suchas cysteine and homocysteine are essential biomolecules required for the normal growth of

cells and tissues in living systems.[36�38] These biological thiols play a significant role in

living systems by providing important insights into the proper physiological functions and

diagnosis of diseases. For instance, a variance in normal levels of cysteine is implicated

with several health problems such as slowed growth, hair depigmentation, oedema,

lethargy, liver damage, muscle and fat loss, skin lesions, and weakness.[39,40] Abnormal

levels of homocysteine in human plasma have been implicated in incidences of

cardiovascular and Alzheimer’s diseases.[39,40] Therefore, it is of great interest to developa selective and sensitive method for the detection of thiol-containing amino acids in

biological samples.

Typical methods for the detection of sulphur-containing amino acids include

high-performance liquid chromatography,[37,40,41] mass spectrometry,[42] capillary

electrophoresis,[41] electrochemical methods,[39,43] and some colourimetric assays.[5�9]

Most of the reported detection methods need complicated and expensive instruments and

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require time-consuming pretreatment procedures for analysis. Compared to theinstrumental analysis, the colourimetric detection method is more convenient and efficient

for thiol detecting.[5�9,44] In recent years, some colourimetric assays based on cysteine-

induced aggregation of AgNPs have been reported as nanosensor probes for the detection

of cysteine.[5�9,32] However, most of the above detection processes rely on the addition

of polymeric surfactants as capping agents or metal ions to enhance the sensitivity and

selectivity of the AgNP probes.[5�9,32] In the present study, highly stable green-tea-

capped AgNPs have been synthesised and used as colourimetric and optical probe for the

selective detection of cysteine. The absorbance of the AgNPs was quenched efficiently inthe presence of cysteine. The assay for cysteine was not only sensitive and selective but

also simple, cost effective, and environmentally friendly.

2. Materials and methods

2.1. Materials

All the chemicals and reagents used in this study were purchased from commercial sources

and used as received. Green tea (Lipton, USA) was purchased from a local store. High-

purity water (Milli-Q 18.2 MV cm) was used for preparing the AgNPs and stock solutions

throughout this study. All the amino acid solutions and reagents were freshly prepared

and used on the same day.

2.2. Instruments

UV�Vis absorption spectra of the AgNPs were recorded on a Varian Cary 50 BIO

spectrometer. Transmission electron microscopy (TEM) images were recorded on a JEOL

high-resolution transmission electron microscope (HRTEM-JEM-2100F) instrument at

200 kV accelerating voltage. The microscope was equipped with a charge-coupled device (CCD)

digital camera, and a carbon-coated copper grid was used for sample preparation. The

configuration and crystallinity of the AgNPs were determined by X-ray diffraction (XRD)(Rigaku, MiniFlex II), operated at a voltage of 40 kV and a current of 30 mA with Cu Ka

radiation. All the pictures of the AgNPs and cysteine mixtures were taken by Samsung

Galaxy SIII camera (Samsung, USA).

2.3. Preparation of the tea extract

Dry green tea leaves (400 mg) were added to 30 mL water in 150-mL Erlenmeyer flask. The

mixture was vigorously stirred over a magnetic stir plate for 30 min at room temperature

and filtered with Whatman 1 filter paper. The freshly prepared tea extract was then usedfor the synthesis of AgNPs as described below.

2.4. Synthesis of AgNPs using green tea extract

Well-defined silver nanoparticles were synthesised using freshly prepared aqueous

solutions of green tea leaf extract in the following manner.[43] The stock solution of the

tea leaf extract was diluted to 5%, 10%, 20%, 30%, and 40% (V/V) with deionised water.

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Next, 1.00 mL of silver nitrate (1 mM) and 0.25 mL of NaOH (0.10 M) solutions wereadded dropwise in sequence into the dilute tea extract solutions. The contents of the

reaction vessels were combined with gentle swirling, and brown AgNPs were produced

immediately at room temperature under ambient light conditions.

2.5. Detections of amino acid with AgNPs

A solution of 80 mL of the green-tea-capped AgNPs was diluted to 3 mL of the total

volume in a quartz cuvette. To this solution, 5 mL (1 mM) cysteine was added, and the

contents of the solution were mixed by using a disposable pipette. The mixture was kept

3�5 min at room temperature for equilibration prior to UV�Vis analysis. The UV�Vis

absorption spectra of the samples were scanned from 230 to 700 nm. The same procedure

was followed for studying the interaction of the various amino acids and biomolecules

with AgNPs. In addition, the interaction of cysteine with the AgNPs in the presence of70 mMNaCl was studied following the same procedure.

3. Results and discussion

3.1. Synthesis and characterisation of the AgNPs

Colloidal AgNPs have been synthesised using an aqueous solution of green tea leaf extract

as a reducing and capping agent at room temperature and alkaline conditions, in an eco-

friendly manner. The reaction for the synthesis of the AgNPs was carried out in alkaline

media, because stable AgNPs could not be produced under acidic or neutral solutions

(data no shown). It is known that green tea contains various polyphenolic compounds andphytochemicals, the prime constituents being theaflavins.[45] These polyphenolic

compounds can be easily deprotonated under alkaline conditions and serve as both

reducing and stabilising agents.[5,32,46] In this regard, the NaOH solution was added to

provide high alkalinity and enhance the oxidation capacity of AgNO3. The formation of

AgNPs was indicated by the immediate colour change from light yellow to dark brown

upon the addition of 0.10 M NaOH. The effect of the amount of tea leaf extract on the

formation of AgNPs was also studied. The amount of silver nitrate was kept constant

(1.0 mL, 1 mM), and the amount of tea extract was varied as 5%, 10%, 20%, 30%, and40% (V/V). The intensity of the brown colour increased with the increase in the

concentration of the tea extract (Figure 1(b)).

As described earlier, longer reaction time is one of the major drawbacks for most of the

plant-extract-based synthetic methods. In most cases, the reaction ranges from 2 to 5 h

and involves continuous stirring and heating. In contrast, this study demonstrates rapid

synthesis of the AgNPs completed within 10�15 min, and confirmed by the instantaneous

colour change from light yellow to brown. The colour change indicates the reduction of

AgC ions to AgNPs under alkaline conditions. Furthermore, this green synthesis ofAgNPs proceeds without the need for any external heating or stirring. Thus, this technique

proves to be simple, inexpensive, and efficient, since no heat or any additional surfactants

or polymers as stabilising agents are needed.

The formation of the tea-extract-stabilised AgNPs was also confirmed by UV�Vis

spectroscopy. The prepared AgNPs exhibited a characteristic surface plasmon resonance

band centred around 410 nm, an absorption wavelength that is insensitive to the

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concentration of tea leaf extract. As shown in Figure 1(b), although different

concentrations of tea extract gave different colours, no shift was observed in the λmax

values of 5%�40% tea leaf extracts (Figure 1(a)). This indicates that nearly the same sizes

of AgNPs are formed regardless of the concentration of the tea extract. However, the

number of AgNPs formed at a high concentration of tea extract is higher than that of a

lower concentration, as evidenced by higher absorbance values. Reported examplesindicate that the increase in absorbance values with the increase in the concentration of tea

extract demonstrates higher production of AgNPs.[10,47] The symmetry of the plasmon

resonance absorption bands as shown in Figure 1(a) also indicates the formation of mostly

uniform AgNPs with low levels of agglomeration in solution.[48]

The formation of the AgNPs was further confirmed by TEM. Figure 2 shows TEM

images of AgNPs synthesised using 20% green tea leaf extract (V/V%). The as-synthesised

particles are predominately spherical in shape with very few having irregular shapes of flat

crystals (Figure 2). The size of the particles was 5�30 nm.The crystalline nature of the AgNPs was also confirmed by XRD. The XRD patterns

of the dried samples of the AgNPs are shown in Figure 3. The diffraction peaks at

2u degrees of 38.02, 44.12, 64.36, and 77.36 can be attributed to the reflections of (111),

(200), (220), and (311) planes of the face-centred cubic crystalline structure of metallic

silver, respectively. In addition, the minor peak near 32.10� implied the possible existence

of Ag2O in the sample.[17,49]

3.2. The stability of the AgNPs

The synthesised AgNPs display excellent stability over a period of more than six months.

When the AgNPs were left standing at room temperature without any specific protection,

no change in UV�Vis absorption spectra was observed (Figure 4(a)). In addition, there

Figure 1. (Colour online) (a) UV�Vis absorption spectra of colloidal solution of AgNPs withdifferent concentrations of green tea leaf extract (5%�40%). (b) Visual images of the AgNPs withdifferent concentrations of tea leaf extracts at room temperature.

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was no discernible colour change in the vials of the as-prepared AgNPs over the six months

period (Figure 4(b)). This stability could be ascribed to the presence of the deprotonated

polyphenolic compounds in the tea extract which act as both stabilising and effective

reducing agents.[32,44�46] Furthermore, UV�Vis spectra of the AgNPs taken after eight

months (Figure 5) also look very similar to the UV�Vis spectra of the freshly prepared

Figure 2. TEM image of 20% green-tea-extract-capped AgNPs.

Figure 3. XRD pattern of AgNPs synthesised using 20% (V/V) tea extract

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nanoparticles (Figure 1). This indicates the high stability of the nanoparticles as described

above. The only difference that we observed was a slight shift in the maximum wavelength

towards the red spectrum.

However, in the presence of 100 mM NaCl, a significant decrease in the plasmon

resonance band centred at 410 nm (Figure 6) was observed. The addition of the salty

solution also induced a noticeable colour change from brown to grey of the colloidalAgNP solution (data not shown). This indicates faster aggregation of the AgNPs in a

strong salty medium. Nevertheless, even here, this occurred after 24 h in the salty solution.

Figure 4. (a) UV�Vis absorption spectra of colloidal solution of AgNPs with differentconcentrations of green tea leaf extract (5%�40%) after six months. (b) Visual images of the AgNPswith different concentrations of tea leaf extracts after six months at room temperature.

Figure 5. (a) UV�Vis absorption spectra of colloidal solution of AgNPs with differentconcentrations of green tea leaf extract (5%�40%) in eight months. (b) Visual images of the AgNPswith different concentrations of tea leaf extracts in eight months at room temperature.

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3.3. Detection of cysteine using AgNPs

The effect of cysteine on the freshly prepared AgNPs was studied. As shown in Figure 7(a),

the addition of increasing amounts of cysteine to the aqueous solutions of the AgNPs

Figure 6. UV�Vis absorption spectra of colloidal solution of AgNPs in the presence of 100 mMNaCl with different concentrations of green tea leaf extract (5%�40%); the stability was measuredafter 24 h. (Note the significant decrease in absorbance at 410 nm.)

Figure 7. (a) Absorption spectra of AgNPs upon addition of increasing amount of L-cysteine(0�60 mM). Arrows indicate spectral changes with increasing concentration of L-cysteine.(b) Calibration curve for the detection of L-cysteine (0�60 mM).

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caused a significant decrease of the absorption band at 406 nm while a new absorptionpeak appeared at 506 nm. The addition of cysteine also induced a distinct colour change

from yellow to pink that enabled the colourimetric detection of cysteine (Figure 8(b)). It is

well known that thiol groups of cysteine interact strongly with AgNPs and are easily

immobilised on the surface of the AgNPs through the formation of Ag�S bonds.[50]

Hence, the resulting change in colour and shift in the wavelength clearly indicated the

cysteine-mediated aggregation of the colloidal AgNPs through hydrogen bonding and

electrostatic interactions.

The selectivity of the AgNPs for cysteine was further evaluated by monitoring thechanges in the ratio of the absorbance at 506 and 406 nm, R (A506/A406). As shown in

Figure 7(b), a good linear relationship exists between the ratio of A506/A406 and the

concentration of cysteine in the 0.0�60 mM range. It is clearly seen that the absorbance

was dependent on the quantity of cysteine added. The new absorption band at 506 nm

became more intense with the increasing concentration of cysteine. Additionally, the

minimum detectable concentration of cysteine based on the naked-eye evaluation of

the colour change corresponded to an approximate concentration of 50 mM. The

corresponding colour variation indicates that tea-leaf-capped AgNPs can potentiallybe used for the detection of cysteine through UV�Vis spectroscopy or by naked-eye

observation.

The detection of cysteine by the AgNPs was also investigated in the presence of 70 mM

NaCl (Figure 9). As shown in Figure 9, although the detection of the cysteine was also

possible in the presence of 70 mM NaCl, the change in the absorbance was not as

significant as without NaCl. This probably shows that in the presence of NaCl, the AgNPs

aggregate rapidly, potentially minimising their effectiveness.

Figure 8. (Colour online) (a) UV�Vis absorption spectra of AgNPs in the absence (green colour)and presence (pink colour) of L-cysteine. (b) Visual images of the AgNPs with or without L-cysteine.

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3.4. Specificity of the cysteine probe

The interference of other relative amino acids and biomolecules on monitoring cysteine

was investigated (Figure 10). The results clearly show that the synthesised AgNPs possess

high selectivity towards cysteine in the presence of other amino acids. Notably, little

spectral changes occurred in the presence of other amino acids and biomolecules. In

addition, other amino acids had no effect on the colour of the AgNPs. Therefore, the

results showed that the AgNPs can be used as simple probes for visualising the presence of

cysteine selectively over other amino acids at room temperature. Remarkably, even thepresence of methionine and histidine amino acids showed no immediate significant colour

or absorbance changes upon addition to the colloidal AgNP solution.

4. Conclusions

In summary, colloidal silver nanoparticles can be prepared by a green synthetic approach,

which involves reduction of silver nitrate using green tea extract under alkaline conditions.The as-prepared AgNPs exhibit high stability and interesting optical properties. The as-

prepared AgNPs could be utilised as simple colourimetric and optical probes for the

selective detection of cysteine over other amino acids including methionine and histidine.

This selective detection occurs due to the specific cysteine-induced aggregation of the

AgNPs through the Ag�S covalent bond formation and electrostatic interaction between

the COO¡ and NH4C zwitter ions and the thiol groups of cysteine.

Figure 9. UV�Vis absorption spectra of AgNPs upon addition of increasing amount of L-cysteine(0�60 mM) in the presence of 70 mM of NaCl. Arrows indicate spectral changes with an increasingconcentration of L-cysteine.

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Acknowledgments

We thank Dr. Wei Cao for the collection of TEM data, and Dr. Silvina Pagola for the XRD datacollection and analysis.

Disclosure statement

The authors declare no competing financial interest.

Funding

This work was supported by the National Science Foundation-Faculty Early Career Development(CAREER) Award [grant number CHE-1230357] and ACE Implementation Award NSF HRD[grant number 1238838].

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