GOLD NANOPARTICLE IN DOPAMINE

BIO-NANOSENSORS

SUBMITTED BY

A.Nakkiran

CECRI

ABSTRACT

Sensor technology is one of the most important key technologies of the future with

a constantly increasing number of applications, both in the industrial and in the private

sectors. More and more bio sensors are used for the control of processes in environment

monitoring, healthcare, and Military (against bioweapons). Consequently, the

development of fast and sensitive bio sensors is the subject of intense research, propelled

by strategies based on nanoscience and nanotechnology. This paper highlights the recent

developments and reflects the impact of nanoscience on electrochemical sensor

technology by elucidates size dependent sensitivity of gold electrode to sense dopamine,

an important neurotransmitter.

Introduction

According to the modern definition, biosensors are analytical devices comprising

a biological or biologically-derived sensing element either integrated within or intimately

associated with a physicochemical transducer1 (Fig1). The transducer is an important

component in a biosensor through which the measurement of the target analyte(s) is

achieved by selective transformation of a biomolecule-analyte interaction into a

quantifiable electrical or optical signal. A wide range of optical and electrochemical

instruments have been employed in conjunction with biological sensing.

Figure 1 Schematic diagram showing the main components of a biosensor

(a) A bio-component, (b) a transducer which converts the biochemical reaction into a

physical signal, (c) an amplifier which converts a physical signal into an electrical signal,

which is processed and displayed by a recorder or PC (d).

Electrochemical transducers detect an electrochemical signal that is generated by the

interaction between the analyte and the receptor. It could be a change of a redox potential

(Potentiometric), the conductivity of the solution (conductometric) or the production of

redox active molecules that generate a current (voltammetric, amperometric,

coulometric).

Dopamine and Electrochemistry

Dopamine is an important neurotransmitter because it is involved in physical and

cognitive functions. To understand the challenges associated with measuring dopamine it

is important to understand the environment in which dopaminergic neurons function.

Neuron Function

• Nerve cells, or neurons, are the basic building blocks of the nervous system. The

neuron is responsible for sending and receiving nerve impulses or messages. A neuron

that is excited will transmit its energy to neurons that are next to it (Fig.2).

Figure 2: Transformations message through neurons

•Neurons have a central cell body attached to slender, branching arms. There are

two types of “arms”: dendrites are like antennae and carry messages, or impulses, to the

cell body, while axons carry messages away from the cell body.

• Impulses travel from neuron to neuron from the axon of one cell to the dendrites

of another by crossing over a tiny gap between the two nerve cells called a synapse.

• Incoming messages from the dendrites are passed to the end of the axon where

sacs containing neurotransmitters (dopamine) open into the synapse.

• The dopamine molecules cross the synapse and fit into special receptors on the

receiving cell.

• That cell is stimulated to pass the message on

• After the message is passed on, the receptors release the dopamine particles

back into the synapse where the excess dopamine is “taken up” or recycled within the

releasing neuron

So inadequately stimulation of dopamine (DA) can cause fatal disease such as

Parkinson’s and schizophrenia. Recent clinical studies have demonstrated that the

content of dopamine in biological fluids can be used to assess the amount of oxidation

stress in human metabolism and excessive oxidative stress has been linked to cancer,

diabetes mellitus, and hepatic disease.

Electrochemical Detection

Dopamine (DA), the most important among the class of neurotransmitters, plays

an important role in the function of the central nervous system. The development of

methods for dopamine quantification in nerves and biological fluids is the subject of

intense current investigation in neurochemical studies.Electrochemical method is an ideal

choice for the quantitative determination of dopamine, because

•Dopamine is easily oxidizable

•R. MARK WIGHTMAN Research group in University of North Carolina has

employed in-vivo voltammetry to measure the dopamine release and uptake in freely

moving animals and found that a behavioral stimulus can evoke a transient increase in

dopamine, providing how a neurotransmitter controls behavior on second and sub second

timescales and revealing how critical rapid, selective, and sensitive measurements for

real-time detection of chemical changes in the brain(Fig 3)

Figure 3 X-Ray image of microelectrode in brain

Drawbacks

But Interference due to the co-existence of ascorbic acid (AA) in the biological

fluids, which also undergoes oxidation more or less at the same potential, is the major

flaw in dopamine sensor. Also, the concentration of AA is relatively higher than that of

DA in these samples (103 times higher than DA), which results in poor selectivity and

sensitivity for DA detection.So the detection of DA in the presence of excess of AA is a

challenging task in electro analytical research.

EXPERIMENTAL SECTION

Chemicals

Hydrogen tetrachloroaurate, dopamine (DA) and ascorbate (AA) were obtained

from Aldrich and were used as received. All other chemicals used in this investigation

were of analytical grade and were used without further purification. The phosphate buffer

solution (PBS) was prepared from NaH2PO4 and Na2HPO4 (0.1 M).

Preparation of Au colloids

Au colloids were prepared according to the literature. Typically, 1 ml of 1%

HAuCl4 was added to 90ml of water at room temperature. After 1 min of stirring, 2 ml of

38.8 mM sodium citrate was added. Subsequently,1 ml of freshly prepared 0.075%

NaHB4 in 38.8mM sodium citrate was added and the colloidal solution was stirred for

another 5-10 min and stored in a dark bottle at 4 8C. The concentration of the Au

nanoparticles was estimated to be 0.32 mM.

Immobilization of Au colloidal particles

The Au electrodes of 1.6 mm diameter were polished with alumina powder (1.0

and 0.06 mm) and sonicated in water for 5-10 min. The polished electrodes were then

electrochemically cleaned by potential cycling between /0.2 and 1.5 V at a scan rate of 10

V s-1 in 0.05 M H2SO4 for 10 min or until the cyclic voltammogram characteristic for a

clean Au electrode was obtained. The electrochemically cleaned Au electrode was

immersed into an aqueous solution of 10 mM of amine-terminated monolayer of

cystamine (CYST) for 1 h. The CYST monolayer-modified electrode was rinsed well

with water and kept in water for at least 30 min to remove the physically adsorbed CYST.

The CYST electrode was subsequently soaked in the Au colloidal solution for 12 h. The

resulting electrode was washed with copious amount of water and subjected to

electrochemical experiments. Here after the Au nanoparticle-immobilized electrodes will

be referred as the nano-Au electrode (fig.4).

Figure 4 Schematic representation of the fabrication of the nano-Au self-assembly (note

that this is a pictorial representation and is not on the correct scale).

1) Bare gold electrode 2) CYSTMINE modified –Au electrode 3) Au nanoparticle

immobilized electrode 4) TEM of Au nanoparticle immobilized electrode

RESULT AND DISSCUSSION

Fig. 5 shows the SW voltammograms obtained for DA at the bare and nano-Au

electrodes. The voltammetric response at the bare electrode is rather broad, whereas it is

sharp and well defined at the nano-Au electrode, suggesting that the electrochemical

behavior of the nano-Au electrode is quite different from the bulk Au electrode.

Furthermore, as can be readily seen from this figure, the peak current at the nano-Au

electrode is significantly larger compared to that of the bare electrode. For instance a

1.7fold enhancement in the peak current at the nano-Au electrode was observed, which

indicates that the nano-Au electrode possesses excellent sensitivity towards DA.

Fig. 5 Square-wave voltammograms obtained for the oxidation of DA (50 mM) at

the bare Au (a) and nano-Au (b) electrodes in 0.1 M PBS (pH 7.2).

Since ascorbic acid (AA) is the major interferent in the voltammetric

measurement of DA, its voltammetric behavior at the Au nano-assembly was studied.

Fig. 6 shows the cyclic voltammograms obtained for the oxidation of AA at the nano-Au

electrode at different scan rates. At the nano-Au electrode the oxidation occurs at around

0.03 V and an enormous increase in the peak current compared to the bare electrode was

observed. These results indicate that the nano-Au electrode effectively catalyzes the

oxidation of AA. It has been reported quite recently that nanometer-sized Au particles

exhibit excellent electro catalytic activity.

Fig. 6 Cyclic voltammograms obtained for the oxidation of AA (100mM) at the nano-Au

electrode in 0.1MPBS (pH 7.2). Scan rate: 25, 50, 75, 100, 125, 150 and 175 mV s_1.

The important attribute of the nano-sized catalysts is the

• High surface area and

•Interface-dominated properties that differs from the atomic, molecular and bulk

counterpart.

In the present investigation the facilitated oxidation of ascorbic acid (AA) at the

nano-Au electrode is believed to be due to the excellent catalytic activity of nano-sized

Au particles. Because the main objective of the present investigation is the determination

of DA in the presence of AA, our attention is focused on the voltammetric detection of

DA in the presence of AA.

Figure 7 Cyclic voltammograms of a binary mixture solution of AA and DA at the bare

Au (a) and nano-Au (b) electrodes (pH 7.2). Scan rate: 100 mV s-1. (B) Corresponding

square-wave voltammograms obtained at bare Au (a) and Nano-Au (b) electrodes.

Fig. 7 shows the cyclic and SW voltammograms obtained for DA and AA coexisting at

bare and nano- Au electrodes. We can see that the bare electrode cannot separate the

voltammetric signals of AA and DA. Only one broad voltammetric signal was observed

for both analytes and the voltammetric peak decreased in the subsequent sweeps.

Therefore it is impossible to use the bare electrode for the voltammetric determination of

DA in the presence of AA. But, the nano-Au electrode resolved the mixed voltammetric

signals into two well-defined voltammetric peaks at 0.015 and 0.185 V corresponding to

the oxidations of AA and DA, respectively. The nano-Au electrode shows good

selectivity and excellent sensitivity in the detection of DA in the presence of AA.AA is

readily oxidized well before the oxidation potential of DA is reached. Thus the catalytic

oxidation of AA by the oxidized DA is completely eliminated and the precise

determination of DA in the presence of AA is possible at the nano-Au electrode. The

voltammetric signals of AA and DA remained unchanged in the subsequent sweeps,

indicating that the nano-Au electrode does not undergo surface fouling. Furthermore, the

separation between the voltammetric peaks of AA and DA is large (165 mV) and thus the

simultaneous determination of AA and DA or the selective determination of DA in the

presence of AA is feasible at the nano-Au electrode

Insights on Influence of Particle Size on Electrochemical bio-sensing

The result shows that nano-sized Au is largely different from the bulk counterpart

and it shows a surprisingly high electro catalytic activity. The nano-Au electrode

successfully distinguishes the voltammetric signals of AA and DA, which are

indistinguishable at the bare Au electrode. Because each incorporated nanoparticle

operate as an individual electrode (electrode of nanosize), and act as an active site for

interfacial electron transfer. Electron transfer at nanoscale electrodes is much different

from that of bulk, because reducing the electrode size increases the diffusion-controlled

transport rate in steady-state voltammetric measurements. Consider the simple electron-

transfer reaction

Ox + e– � Red

Occurring at a spherical electrode of radius a, Diffusion of Ox to the electrode

surface occurs before the electron-transfer step, and either step may be rate-limiting. At

steady state, the rate constant for diffusion (cm/s) of redox molecules to the spherical

electrode is simply D/a, in which D is the diffusion constant (cm2/s) of Ox. Since a is in

nanometer, D/a is comparable to or significantly larger than the standard electron-transfer

rate constant ket (cm/s).

D/a ≥ket

So the overall rate of the electrode reaction is solely controlled by the interfacial

electron-transfer step (i.e. transport rate of analyte from the bulk of the solution to the

interface is not accounted).Hence nanoelectrode open up possibilities for work in very

low concentration (nanomolar) of analyte.whatever can be done at a planar electrode can

be done at concentration of about 106 times lower by using nanoelectrode without

reaching the limiting current. But in bulk electrode the reaction is under diffusion control,

so low concentration analyte reactions are not easily distinguishable from the high

concentrated interference molecule. Hence reason for the high sensitivity of

nanoelectrode is well explained.

CONCLUSION

The present work reveals the fact that; size miniaturization is the principal

cause for high selectivity and high sensitivity of DA sensor. Also proposed reason

soundly corroborates the obtained result. Thus the potential application of nano-

sized Au for the fabrication of a voltammetric DA sensor is demonstrated.