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Prototypical Nonelectrochemical Method for Surface Regeneration of anIntegrated Electrode in a PDMS Microfluidic ChipXuan Mu ab; Qionglin Liang b; Ping Hu c; Bo Yao b; Kangning Ren b; Yiming Wang b; Guoan Luo ab

a School of Pharmacy, East China University of Science and Technology, Shanghai, China b Key Laboratoryof Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry,Tsinghua University, Beijing, China c College of Chemical and Molecular Engineering, East China Universityof Science and Technology, Shanghai, China

Online Publication Date: 01 January 2009

To cite this Article Mu, Xuan, Liang, Qionglin, Hu, Ping, Yao, Bo, Ren, Kangning, Wang, Yiming and Luo, Guoan(2009)'PrototypicalNonelectrochemical Method for Surface Regeneration of an Integrated Electrode in a PDMS Microfluidic Chip',AnalyticalLetters,42:13,1986 — 1996

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ELECTROANALYTICAL

Prototypical Nonelectrochemical Methodfor Surface Regeneration of an IntegratedElectrode in a PDMS Microfluidic Chip

Xuan Mu,1,2 Qionglin Liang,2 Ping Hu,3 Bo Yao,2 Kangning Ren,2

Yiming Wang,2 and Guoan Luo1,2

1School of Pharmacy, East China University of Science and Technology,Shanghai, China

2Key Laboratory of Bioorganic Phosphorus Chemistry and ChemicalBiology (Ministry of Education), Department of Chemistry,

Tsinghua University, Beijing, China3College of Chemical and Molecular Engineering, East China University

of Science and Technology, Shanghai, China

Abstract: A prototypical method for surface regeneration of an integratedelectrode in a microfluidic chip is demonstrated. A platinum wire as workingelectrode was mounted in a polydimethylsiloxane chip vertical to the chip throughthe channel. The regeneration of the electrode was easily achieved by drawing theplatinum wire out for 5mm, because the area exposed to the channel or thestream would be altered. With continuous motion, the wire electrode can main-tain a fresh surface just like a dropping mercury electrode. The current–time curveand open circuit potential (OCP) of dopamine solution show the performance ofthis prototypical system.

Received 18 November 2008; accepted 19 May 2009.This research was supported by funding from the National Basic Research

Program (973 Program) of China (2007CB714505) and the Ministry of Educationof China (No. 20080031012).

Address correspondence to Guoan Luo and Qionglin Liang, Key Laboratoryof Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Educa-tion), Department of Chemistry, Tsinghua University, Beijing 100084, China.E-mail: [email protected]; [email protected]

Analytical Letters, 42: 1986–1996, 2009Copyright # Taylor & Francis Group, LLCISSN: 0003-2719 print=1532-236X onlineDOI: 10.1080/00032710903082762

1986

Downloaded By: [University of California San Diego] At: 11:07 21 September 2009

Keywords: Dopamine, electrochemistry detection, microfluidic chip, PDMS,surface regeneration

INTRODUCTION

Electrochemical (EC) detection integrated in microfluidic chips hasdeveloped rapidly in recent years (Wang 2005;Mora et al. 2008; Vandaveeret al. 2004; Xu, Wang, and Chen 2007). Although laser-induced fluores-cence (LIF) is the most prevalent detection method for microfluidicdevices and miniature LIF configuration has been developed (Yao et al.2006, 2005; Ren et al. 2007, 2009), EC detection still has inherent advan-tages, such as miniaturized detectors and control instruments. There areother advantages of EC, including high sensitivity (approaching that ofLIF), no need for derivatization, and less cost. Until now, there have beenthree methods to integrate an electrode with a microfluidic chip:

1. The electrode (band) is fabricated in the chip by standard MEMS(micro-electro-mechanical systems) processes (Woolley 1998; Martin2000; Yan 2003; Joo 2007; Wang 1999). It is beneficial that the micro-fabrication of the electrode is compatible with the production ofmicrochip.

2. The electrode (wire, strip) is mounted perpendicularly to the flowdirection at the channel outlet or inserted into the channel (Fanguy2002; Zeng 2002; Xu 2004; Qiu 2007; Martin 2002). In this configura-tion, the electrode is less expensive and could be replaced more easily.

3. The electrode (wire) is put into the electrode channel through theseparate channel transversely perpendicular to the latter (Garcia andHenry 2003; Liu, Vickers, and Henry 2004). This configuration pro-vides greater collection efficiency for the analyte. As far as we know,the lowest detection limit of dopamine (600 pM) was achieved by thisconfiguration (Vickers, Caulum, and Henry 2006).

Electrochemical (EC) detection suffers from the fact that somematerials are absorbed on the electrode surface, which would cause a lossof sensitivity (Manica, Mitsumori, and Ewing 2003). So, regenerating theelectrodes and regaining a new surface is crucial in electrochemical detec-tion. For integrated microelectrodes, tens of cyclic voltammograms (CVs)were usually carried out to regenerate the surface. It is simple to accom-plish; however, the regeneration could not be done simultaneously withelectrochemical detection. Another method, the so-called pulsed electro-chemical detection (PED), is also developed to maintain surfacecondition (LaCourse and Modi 2005) (Garcia and Henry 2005). The

Nonelectrochemical Method for Surface Regeneration 1987


PED method employs three distinct potentials. The first potential is forcleaning, the second potential is for regeneration, and the third one isfor detection. It has been successfully applied to detection of carbohy-drates, thiols, and amino acids; however, this method is much more com-plicated to manipulate and optimize. Both an expensive instrument andhigh skill level are therefore needed (Garcia and Henry 2005). The men-tioned techniques, CV and PED, are electrochemical treatments, so theregeneration of electrode inevitably disturbs the electrochemical detectionand is inherently inefficient.

In the present article, a nonelectrochemical manner to regenerate theelectrode surface is described. The unique feature of this configuration isthat the solid electrode could change its position in channel. The methodseems like a hybrid of a solid electrode and a dropping mercury electrode(DME). There are three advantages to this methodology: it is (1) indepen-dent of electrochemical detection, (2) able to regain a brand-new surface,and (3) easy to implement. This method was tested with current–timecurve and open circuit potential (OCP). The experiment proved thatthe surface can be regenerated effectively.

EXPERIMENTAL

Reagents and Chemicals

All reagents used were of analytical grade, and Milli-Q water (18.2MX;Millipore, MA) was used throughout the experiment. Fluorescein iso-thiocyanate (FITC) (Sigma, USA; 1mM) was prepared by dissolving0.038 g FITC in 100ml water. Potassium ferrocyanide solution (0.5mM)was prepared in 100ml KCl (0.1M) solution. Dopamine (Sigma, USA)solution (2mM and 200 mM) was prepared in 50ml KCl (0.1M) solution.Microwires (100 mm) made of 99.95% platinum were purchased from AlfaAesar (MA, USA).

Fabrication of Microfluidic Chip

Photolithographic and wet chemical etching techniques were used forfabricating positive relief patter onto a 1.7-mm-thick 63-� 63-mm glassplate with chromium and photoresist coating (Shaoguang Microelectro-nics Corp., Changsha, China) (Yao et al. 2004). The glass with positiverelief was used as a master. The PDMS prepolymer (five monomersand one curing agent) with excessive Si-H group was cast on this masterand cured thermally (75�C). After that, the PDMS plate was peeled from

1988 X. Mu et al.


the master with channels. The reservoirs and cell were punched andtrimmed. This channel plate and another blank plate (20 monomersand one curing agent) with excessive vinyl group were bonded togetherthermally to form an irreversible seal. The channel is 250 mm wide,20 mm deep, and 2.1mm long. The chip is 5mm thick (Fig. 1a). The sam-pling channel for electrophoresis is simplified and omitted.

Electrode Integration

A piece of glass capillary was pulled by an alcohol blast burner to maketip of glass around 300 mm wide. A platinum wire (100 mm) was placed in

Figure 1. Schematic diagram of surface regenerable electrode in a PDMS chip. 1electrochemical station; 2 Inlet; 3 PDMS microchip; 4 Working Electrode (WE); 5Counter electrode (CE); 6 Reference electrode (RE); 7 Cell; 8 the copper wire(1mm) connected with Pt wire and electrochemical station; 9 stretched glass capil-lary. (a) The channel is 20j�mu�jm deep, 2.1 mm long and 250j�mu�jm wide. WEis a 100-j�mu�jm Pt wire, at the end of channel and piercing it. The distance formWE to WR is about 2mm. RE is a saturated calomel reference electrode. (b) ThePt wire was inserted in 9 (stretched glass capillary). (c) Fluorescent image of 1mMFITC as it passes the working electrode. Leakage of FITC was not observedaround the Pt wire in experiment. (d) Microscope image of work electrode piercedin the channel. The scale bar represents 100j�mu�jm.



the capillary (Fig. 1b). The capillary was aligned to the channel and waspierced through the chip. Then the capillary was drawn off while theplatinum wire was left in the chip (Figs. 1c and d). After the work elec-trode was ready in place, a copper wire (1mm) was soldered to the endof work electrode and held in position with epoxy glue. The copper wirewas not fixed but curved to certain angle to adjust the position of theworking electrode.

The chip was designed to allow the working electrode to regenerate.To determine if fluid leaked from the work electrode, 1mM FITC waspumped into the channel to visualize it when it passes the work electrode.The fluorescent image was acquired by a color CCD camera (DH-HV1302UC, Daheng Image, Beijing, China) coupled with a televisionzoom lens (MLM3X-MP, Computar).

Electrochemical Detection

All reagents were slightly pumped into the channel. Electrochemicaldetection of dopamine and K4Fe(CN)6 were conducted by an electroche-mical station (CH660A, CH Instruments). The scan range of the cyclicvoltammetry was from �0.2 to 0.7V, and scan rate was variable incertain situations. Initial potential of amperometric was 0.45V. The satu-rated calomel reference electrode and one Pt rod (1mm diameter) as acounterelectrode were put into the cell downstream.

RESULTS AND DISCUSSION

The regeneration of electrode surface is essential to electrochemicaldetection, because the accumulation of absorbed oxide would foul theelectrode and lead to the loss of sensitivity. For the integrated electrode,electrochemical treatment to regenerate the surface prevailed. However,tens of CVs hardly get rid of the absorbed oxide and do not retain thesurface as brand-new (Manica, Mitsumori, and Ewing 2003); PED needsoptimization of the parameters and complicated instrumentation(LaCourse and Modi 2005).

The system described here is a prototypical method, a nonelectro-chemical treatment to regenerate the surface of the integrated electrode.The Pt wire as work electrode was pierced into the PDMS channel.The elasticity and hydrophobicity of PDMS prevented leakage fromchannel. The surface of the electrode passed the channel consecutively,analogous to the dropping mercury electrode (DME) (Fig. 2). Thesurface of mercury drop is always changing and maintained fresh.

1990 X. Mu et al.


We simply draw the electrode out by 5mm to achieve the same resultas the DME. The schedule to regenerate the electrode is simple and isdescribed here. Before analysis, the surface of electrode is new and fresh(Fig. 2a). By hydrodynamic or electrokinetic driving, the analyte wouldreach the electrode. When the analyte touched the electrode, electro-chemical reaction occured on the surface (Fig. 2b). As the oxidation per-sisted, the oxides were absorbed on the surface and fouled the electrode(Fig. 2c). It ruined the sensitivity. The Pt wire was drawn manually for5mm so that the surface of electrode exposed to fluid was altered, andthe electrode was regenerated as well (Fig. 2d). The whole system wasready for the next analyte. This configuration also shows that the noblemetal electrode would be recycled after the microchip is discarded. Thedistance to pull the wire out, 5mm, was decided by several parallel tests:a shorter distance cannot provide complete regeneration and a longer onewastes too much Pt wire and causes unnecessary electroresistance.

The surfaces of Pt wire and PDMS are hydrophobic, and the contactangle of water is 114� and 98� respectively (Fei, Chiu, and Hong 2008;Huang et al. 2006) (Fig. 3a). (If the inner wall was processed to be hydro-philic, the fluid tended to flow through the channel rather than leak fromthe gap.) The Young’s module of PDMS is about 750 KPa, so the elastic

Figure 2. Schematic diagram showing the regeneration steps of solid wire elec-trode. (a) Before analysis, the surface of electrode is cleaned and fresh. Diamondrepresents the analyte. By hydrodynamic or electrokinetic driving, the analyte willreach to the electrode. (b) When analyte reach the electrode, the electrochemicalreaction occurs on the electrode surface. (c) The accumulation of oxides on thesurface fouls the electrode. Pentagram represents the oxide. Arrow indicates thedirection of electrode movement. (d) Draw the Pt wire by a little distance and thusthe electrode surface is regenerated. (e)–(h) illustrates continues motion ofelectrode. The curve of copper wire is changed manually to drive the motionof electrode.



force was exerted against the Pt wire when it was inserted (Fig. 3b).Because the elastic force depended on deformation, it was relativelysmall, as the deformation caused by a Pt wire was small, but it wasenough to prevent leakage. Leakage was not observed throughout theexperiments. This method seems to be feasible for other elastic material.

To validate this hypothesis, the K4Fe(CN)6 solution was pumpedgently into the microfluidic chip. The cyclic voltammetric measurementof 0.5mM K4Fe(CN)6 in 0.1M KCl solution was performed routinelyto determine the health of the electrochemical detection system. TheCVs were conducted at various scan rates (Fig. 4a). The oxide peakcurrent was linearly proportionally to the square root of scan rate, ran-ging from 10 to 40mV=s (Fig. 4a inset). It was in accordance with theRandle–Sevick equation (Bard and Faulkner 2000):

iP ¼ 2:69� 105n3=2AD1=2v1=2c

where ip is the peak current; n is the number of moles of electrons trans-ferred per mole of electroactive species (e.g., ferrocyanide); A is the areaof the electrode in cm2;D is the diffusion coefficient in cm2=s;C is the solu-tion concentration in mol=L; and v is the scan rate of the potential in V=s.

Dopamine is an important neurotransmitter that affects manyfunctions of brain such as sleeping, mood, and learning. Some technologyhas been developed to analyze dopamine on chip (Garcia and Henry2003; Liu, Vickers, and Henry 2004) or off chip (Winter, Codognoto,and Rath 2007; Wang et al. 2007; Yang et al. 2006). It is obvious that uti-lizing a microchip to detect dopamine has many advantages, such as

Figure 3. Schematic of PDMS of preventing leakage? (a) Hydrophocity ofPDMS. j�angle�ja is contact angle of water on Pt, which is 114�. j�angle�jb is con-tact angle of water on PDMS, which is 98�. (b) Elasticity of PDMS. PDMS isexerted against Pt wire.

1992 X. Mu et al.


speed (high efficiency), low consumption, and practical on-site detection.The amperometirc i–t curve of 2mM dopamine solution was performedat 0.45V (Fig. 4b). The electrode was manually drawn out 5mm every50 s. The current rose dramatically as the electrode was pulled but slowlyfell and steadied at less than 100 s. The main cause of current jump wasthat the fresh surface of electrode could supply a more effective area.More substrates would be involved in the electrochemical reaction. Thedecrement of current is because the diffusion of the dopamine molecular

Figure 4. (a) Cyclic voltammograms for K4Fe(CN)6 in 0.1M KCl solution at var-ious scan rates. The peak current vs. the square root of scan rate shows linearrelation. The error bar came from at least three parallel experiments. (b)Current-time curve of dopamine (2mM) in KCl solution. Solid curve (—) repre-sents current with electrode regenerating. Ever 40s (the black arrows indicate)electrode was drawn out 5mm. Dash curve (---) represents the current withoutelectrode regenerating. (c) Current-time curve of dopamine (200j�mu�jM) inKCl solution. Solid curve (—) and Dash curve (---) indicate the same as (b); (d)Solid curve (—) represents the current-time curve. Point (�) represents OpenCircuit Potential (OCP). The OCP was measured before and after regeneration.Arrows indicate the regeneration of electrode. The OCP was increased after i–tcurve and decrease after regeneration.



rather than electrochemical kinetics dominates. When there is a balancebetween the diffusion and electrochemical kinetics, the current wassteady. The same experiment was carried out on much weaker dopaminesolution (200 mM), and the results fully met the expectations and con-firmed the observations from previous tests.

The OCP (open circuit potential) was used to characterize the con-dition of the electrode surface. If the electrode is cleaner, the OCP willbe smaller, and vice versa. Three i–t curves under 0.45V detectionpotential were further performed (Fig. 4c). The OCP measurement andi–t curve were not carried out simultaneously. The OCP was recordedbefore and after regeneration. Arrows indicate the motion of theelectrode. After about 200 s at 0.45V, the OCP rose from 0.24 to0.34V, indicating that the dopamine was oxidized and absorbed on thesurface of the electrode; after regeneration, it reduced from 0.34 to0.24V, showing that the electrode was regenerated.

CONCLUSIONS

A prototypical method of electrode surface regeneration was developedand demonstrated. The current–time curve of dopamine solution andthe OCP of the electrode were employed to testify this prototype system.The current of i–t curve and OCP would come back to initial values byregenerating the electrode, giving evidence that the surface was totallyrefreshed. It is just like DME to regenerate the surface of a solid electroderather than liquid one. Manual operation of this system, which may bethe major disadvantage, can be sufficiently solved by a stepwise motor.With this promising method, further research into the detection of thiols,carbohydrates, and complex samples can be conducted in the future.

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