Simple and facile preparation of silver-polydopamine (Ag-PDA) core-shell nanoparticles for selective electrochemical detection of cysteine

Article abstract of DOI:10.1039/c6ra06994k

Selective and sensitive non-enzymatic electrochemical detection of cysteine (CySH) is achieved in the present work using a polydopamine capped silver nanoparticles (Ag-PDA) modified indium tin oxide (ITO) electrode. Efficient redox properties, synergistic effects and the specific steric hindrance associated with the Ag-PDA core-shell nanoparticles provide higher selectivity and larger sensitivity for CySH detection over other competitive bio-thiols namely, homo-cysteine and glutathione. A simple one-step method is used for the preparation of Ag-PDA core-shell nanoparticles. The structure, morphology and composition of Ag-PDA nanoparticles are characterized by using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-visible (UV-vis) and Fourier transform infra red (FTIR) spectroscopic techniques. Electrochemical characteristics are investigated by using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). These studies clearly reveal the formation of Ag-PDA nanocomposite on the ITO electrodes and their corresponding redox properties. Non-enzymatic electrochemical detection of CySH is carried out using Ag-PDA modified ITO electrodes in 0.1 M PBS (pH = 5.0) aqueous solution. Under the optimized conditions, this particular electrochemical biosensor exhibits a perfect linear calibration plot in the concentration range between 0.05 μM and 300 μM. Further, a sensitivity value of 0.023 μA μM^-1 and a lower detection limit of 0.02 μM are determined for CySH detection. Moreover this Ag-PDA modified ITO electrode is applied further for the determination of CySH in human blood serum samples and the results are promising and satisfactory, suggesting the possible analytical application of this biosensor for the determination of CySH in biological samples.

Full text of DOI:10.1039/c6ra06994k

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Simple and Facile Preparation of Silver – Polydopamine (Ag – PDA) Core
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DOI: 10.1039/C6RA06994K
– Shell Nanoparticles for Selective Electrochemical Detection of Cysteine
Raju Thotaa,b and V. Ganesha,b,*
a Electrodics and Electrocatalysis (EEC) Division, CSIR – Central Electrochemical Research
Institute (CSIR – CECRI), Karaikudi – 630003, Tamilnadu, India.
Tel: +91-4565-241242; Fax: +91-4565-227779.
bAcademy of Scientific and Innovative Research (AcSIR), New Delhi – 110025, India.
*Corresponding Author’s E-mail: vganesh@cecri.res.in (or) ganelectro@gmail.com
Abstract
Selective and sensitive non-enzymatic electrochemical detection of cysteine (CySH) is
achieved in the present work using polydopamine capped silver nanoparticles (AgPDA)
modified indium tin oxide (ITO) electrode. Efficient redox property, synergistic effect and
the specific steric hindrance associated with that of Ag–PDA core – shell nanoparticles
provide higher selectivity and larger sensitivity for CySH detection over other competitive
bio-thiols namely, homo-cysteine and glutathione. A simple one-pot method is used for the
preparation of Ag–PDA core – shell nanoparticles. Structure, morphology and composition
of AgPDA nanoparticles are characterized by using field emission scanning electron
microscopy (FESEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD),
X-ray photoelectron spectroscopy (XPS), UVVisible (UV – Vis) and Fourier transform
infra red (FTIR) spectroscopic techniques. Electrochemical characteristics are investigated
by using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). These studies
clearly reveal the formation of AgPDA nanocomposite on ITO electrodes and their
corresponding redox properties. Non-enzymatic electrochemical detection of CySH is
carried out using Ag–PDA modified ITO electrodes in 0.1 M PBS (pH = 5.0) aqueous
solution. Under the optimized conditions, this particular electrochemical biosensor exhibits a
perfect linear calibration plot in the concentration range between 0.05 μM and 300 μM.
Further a sensitivity value of 0.023 μA/μM and a lower detection limit of 0.02 μM are
determined for CySH detection. Moreover this AgPDA modified ITO electrode is applied
further for the determination of CySH in human blood serum samples and the results are
promising and satisfactory, suggesting the possible analytical application of this biosensor
for the determination of CySH in biological samples.
1

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Keywords: Cysteine, core – shell nanoparticles, electrochemical biosensor, in situ
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DOI: 10.1039/C6RA06994K
preparation, non-enzymatic approach, polydopamine, synergistic effect.
1. Introduction
LCysteine (CySH) is one of the most important biological thiols possessing a very
low molecular mass. It plays a key role in cellular metabolism and antioxidation
protection.1,2 Altered levels of CySH in physiological fluids of human beings such as
plasma, urine and saliva results in a number of clinical situations like slower growth, hair
de-pigmentation, edema, lethargy, liver damage, muscle and fat loss, skin lesions and body
weakness etc.13 Monitoring the level of CySH concentration in such physiological fluids
can serve as a valuable biomarker for several clinical diseases and associated applications.
The normal and allowed level of CySH in human body fluids varies between 5 μM and 30
μM and any deviation from this concentration range leads to severe problems as mentioned
above.2 A number of methods have been developed for CySH detection including high
performance liquid chromatography (HPLC), gas chromatography paired with mass
spectrometry, fluorimetry and electrochemistry.1,411 Among these methods electrochemical
detection has attracted a lot of attention due to its low cost, high sensitivity, fast response
time and smaller dimensions of electrochemical sensors (microelectrodes) and also they are
more suitable to be applied for in vivo and on site analyses.1214 However many reports show
that selective detection of CySH using solid electrodes such as platinum (Pt), gold (Au) and
graphite is still very challenging due to their poor electrochemical catalytic behaviour
towards CySH detection. To some extent these problems have been overcome by the use of
redox mediators and chemically modified electrodes with suitable functional materials.
Recently, many different chemically modified electrodes comprising of carbon based
materials like graphene nanoribbon,14 noble metals along with carbon hybrids, multi-walled
carbon nanotubes – gold nanorods,15 multi-walled carbon nanotubes – Au nanoparticles,16
graphene oxide – Au nanoclusters3 and metal organic frameworks17 have been reported for
the detection of CySH. Among the redox mediators catechol group possess a unique redox
activity owing to the presence of hydroquinone / quinone redox system that can mediate
electron transfer reactions across electrode  electrolyte interfaces at a much faster kinetics.
Some of the literature reports state that quinone moiety containing redox active polymers
modified electrode has also been used for sensing of CySH.18,19
2

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Dopamine (DA) is an important catecholamine neurotransmitter consistingViewofArticle Online
catechol groups. It can be oxidized and spontaneously selfpolymerized undDeOrI: a10l.k10a3l9i/nCe6RA06994K
conditions (pH ˃ 7.0) to form polydopamine (PDA) which consists of intrinsic redox moiety
namely quinone within its structure.20,21 In addition DA also has the ability to reduce metal
ions to metal nanoparticles (for example, Au3+ to Au0 and Ag+ to Ag0) and simultaneously
PDA can stabilize these nanoparticles by preventing their aggregation.2224 Particularly the
metal–binding ability of catechol and nitrogen containing groups present within PDA enable
high yields of metal–carbon hybrid materials, which is likely to enhance their applications in
the fields of electrochemistry and photochemistry. Very recently Ag nanoparticles loaded
with PDA spheres are reported for antibacterial applications.25 Moreover hybrid hollow
microspheres of Ag stabilized by PDA are demonstrated to be a potential electrochemical
biosensor for the detection of hydrogen peroxide (H2O2).26 Similarly one–step synthesis of
monodispersed PDA coated silver core – shell nanostructure for enhanced photocatalytic
application is also reported.27
Inspired by these points and the necessity of selective CySH detection, in this work
for the first time to the best of our knowledge, polydopamine capped silver nanoparticles
modified ITO electrode (AgPDA/ITO) is demonstrated for the selective detection of CySH
using an electrochemical method. Particularly intrinsic redox property associated with PDA
and synergistic effect arises from Ag–PDA nanoparticles are explored for the selective and
sensitive nonenzymatic electrochemical detection of CySH. A simple in situ method
without using any additional reducing and / or stabilizing reagents is employed for the
preparation of AgPDA core – shell nanoparticles. Structural and morphological
characterizations of such nanoparticles are analyzed by using FESEM, TEM, XRD, XPS,
UVVis and FTIR techniques. Electrochemical studies, primarily cyclic voltammetry (CV)
and linear sweep voltammetry (LSV) are used for investigating the electrochemical
characteristics of AgPDA nanoparticles modified ITO electrodes (AgPDA/ITO). These
studies clearly reveal interesting electrochemical characteristics associated with that of
CySH oxidation on AgPDA/ITO electrodes. Further the fabrication of nonenzymatic
electrochemical biosensor is carried out by exploring their corresponding (electro)catalytic
activity and several characteristics of such a sensor are determined. These parameters are
much superior when compared to PDA/ITO and Ag/ITO electrodes in terms of increasing
current density associated with the sensing of target analyte, CySH along with a higher
sensitivity and lower detection limit values. This may be due to the enhanced electron
3