Menglun Zhang, Xi Zhang, Pengfei Niu,,a) Tao Shen, Yi Yuan, Yuantao Bai, and Zhilin Wang

ABSTRACT Heavy metal pollution in water environments poses a great threat to public health and to the ecological environment due to its high toxicity and non-degradability.However,many existing detection methods require laboratory-based bulky instruments and time-consuming manual operations. Although some on-site systems exist, they are difficult to deploy on a large scale owing to their large size and high cost. Here,we report a sensing node featuring low power consumption and low cost,achieved by integrating microsensor,microfluidic,and electronic modules into a compact size for automatic and scalable heavy metal pollution monitoring.Digital microfluidic and electrochemical sensing modules are integrated on a chip, thereby combining the procedures of sample pretreatment, electrochemical sensing, and waste removal for automatic and continuous monitoring.The feasibility of the platform is demonstrated by Pb2+detection in tap water.With a 3500 mA⋅h battery,the compact sensing node could work for several years in principle.There is scope for further improvements to the system in terms of wider functionality and reductions in size,power consumption,and cost.The sensing node presented here is a strong candidate for distributed monitoring of water quality as an Internet-of-Things application.

© 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/10.0003511

KEYWORDS Digital microfluidics, Electrochemical sensor, Heavy metal, Node

I. INTRODUCTION

Industrialization and agricultural activities have released a vast number of heavy metal ions into the water environment.1Some of these, such as zinc and copper, are necessary for human metabolism,2,3but others, such as lead and mercury, are harmful to human health and can even cause death.4Therefore, it is very important to be able to effectively and accurately detect heavy metal ion levels in water on a regular basis. Currently, measurements of heavy metals rely mainly on laboratory techniques, including atomic absorption spectrometry (AAS)5,6and inductively coupled plasma mass spectrometry (ICP-MS).7However, these are timeconsuming, expensive, and, more importantly, cannot be implemented in the field for continuous monitoring, which increases the risk of faulty detection caused by sample contamination during transport.Therefore,there is an urgent need for portable analytical tools that can offer on-site and real-time monitoring of heavy metal ions.

Electrochemical sensing approaches fulfill the need for in-field,sensitive, rapid, and inexpensive analysis and require only compact and low-power instrumentation. Integrating electrochemical sensors into microfluidic platforms(i.e.,electroanalytical microfluidics) allows sample preconditioning and analysis to be performed on a small chip, thus providing automated semicontinuous detection.Electroanalytical microfluidic heavy metal detection platforms have attracted much attention from both academia and industry in the past decade.8-10In general,microfluidic platforms can be classified into three categories:microchannel-based microfluidics,paperbased microfluidics, and digital microfluidics (DMF). Heavy metal detection methods employing the first two of these electroanalytical microfluidic platforms have been extensively investigated.11However, channel-based microfluidic techniques require microvalves and micropumps for liquid flow control and are therefore relatively complex in operation and have a high energy consumption.12Paperbased electroanalytical microfluidics techniques actively guide aqueous movement along a paper substrate and have the advantages of extremely low cost,easy fabrication,flexibility,and low energy consumption.They have therefore attracted much attention as tools for heavy metal analysis.13-15However, the water flow rate in paper is hard to control,which makes accurate detection very difficult.Thus,at present,there is no device or system that simultaneously meets all the key requirements for an on-site heavy metal ion sensing node:low power consumption, small form factor, low cost, and accurate fluidic control.

DMF platforms with low power consumption and flexible droplet processing represent a promising solution to the above problems. They are able to perform manipulations of discrete droplets with nanoliter to microliter volumes,including generation,merger(mixing),and separation,on a flat surface with patterned electrode arrays, often via the electrowetting effect.16-18The digital feature of DMF platforms enables sample pretreatments such as quantitative mixing, which makes them very suitable for chemical analysis,19,20biomedical applications,21,22and sample preparation.23,24Their high energy transduction efficiency and the absence of any requirement for extra micropumps and microvalves for fluid control make their power consumption much lower than that of channelbased microfluidics. Moreover, the pathway of droplet transportation is programmable and reconfigurable based on specific requirements, and DMF platforms can be used multiple times. Therefore,electroanalytical microfluidic systems combining DMF platforms and electrochemical sensors are very attractive as sensing nodes distributed at different positions in a water environment to provide frequent monitoring of heavy metal ions. However, to the best of our knowledge, there have been no reports of such systems in the literature.

In this paper, we propose a miniature sensing node for the detection of heavy metals in water environments. It integrates an electrochemical sensor with a DMF platform and allows automatic sample pretreatment, sensing, waste collection, and data acquisition. To match the sensing node, we develop a portable control system,including battery-powered modules and relay control modules. A study of the detection of lead in tap water is carried out to demonstrate the potential of the proposed system.

II. EXPERIMENTAL

A. Fabrication of DMF chip

The DMF chip consists of bottom and top plates.The bottom plate was fabricated on a 500-μm-thick 4-in.silicon wafer by a standard microelectromechanical system (MEMS) process. A 200-μmthick layer of molybdenum(Mo)was sputtered and patterned onto the Si substrate to form square DMF actuation electrodes (2 mm× 2 mm). Then, a 1.1-μm-thick aluminum nitride (AlN) film was deposited as a dielectric layer. Finally, 2% Teflon was spin-coated as an ∼30-nm-thick hydrophobic film onto the DMF and baked at 165○C for 15 min. The dimensions of the chip are 3.5 cm (length)×2.4 cm(width),as shown in Fig.1(a).The top plate serves as the ground electrode of the DMF chip and was made of Teflon-coated indium tin oxide(ITO)glass with dimensions 8.5 cm(width)×1 cm(length). The top and bottom plates were stuck together with 240-μm-thick double-sided tape.More fabrication details can be found in our previous work.25

B. Fabrication of electrochemical sensor

As shown in Fig.1(b),100-nm-thick gold thin film electrodes were patterned onto a polyester film (polyethylene terephthalate,PET)substrate by electronic beam evaporation using stencil copper shadow mask lithography. A 30-nm-thick titanium (Ti) layer was deposited on the substrate before gold growth to improve the interfacial adhesion.The dimensions of the electrochemical sensor chip are 1.2 cm (width) × 2.2 cm (length). The diameter of the working electrode is 1.2 mm. The working area, including working, reference, and counter electrodes, is rectangular, with dimensions 3.6 mm(width)×2.8 mm(length).This size can be integrated well with the DMF platform.The volume of the droplets treated by the DMF platform is 11 μl,which is sufficient to cover the detection area of the electrochemical sensor.

FIG.2.(a)Schematic and(b)photograph of the detection system.(c)Block diagram of control board.(d)Sketch of the DMF electrode pattern and the droplet transport pathway for pretreatment.The red arrows(1)show the pathway for drop mixing and the blue arrows(2)show the pathway for sample transport.(e)Images extracted from a movie showing the progress of the sample as it moves in,undergoes detection,and moves out.

C. Assembly of digital microfluidic electroanalytical platform

The digital microfluidic electroanalytical platform is composed of a DMF chip, an electrochemical sensor, and a liquid absorber.The electrochemical sensor chip was fixed on the outer surface of the ITO glass (top plate) by double-sided tape, with the sensing electrodes outside the ITO glass like eaves on a building and facing downward [Fig. 1(d)]. A liquid absorber was fixed to the bottom plate by double-sided tape,with a separation from the sensing electrodes of about 3 mm.This distance is very important,because it ensures the separation of the 11 μl of liquid once it has been transported by the DMF chip from the absorber.After electrochemical measurement, another drop is transported to the sensing area to enlarge the liquid volume, and coming into contact with the absorber for liquid removal. In contrast to the integration strategy in which the electrochemical sensor is patterned onto the DMF chip itself,26-28with the present system both the DMF chip and electrochemical sensor can be used multiple times, thereby reducing the cost.

D. Control circuit for digital microfluidics

A portable control circuit was designed to process the droplets on the digital microfluidics.As shown in Figs.2(a)and 2(b),it contains a microprogram control unit (STM32F103VC, STMicroelectronics, Switzerland), 60 photo relays (AQW216A, Nais, Japan), a 12 V DC battery,a direct current to direct current(DC-DC)transformer(WRH1290D-8,Nengda,China),a direct current to alternating current (DC-AC) transformer (ICL8038, Renesas, Japan), and an amplifier ICL8038(Renesas,Japan).

E. Electrochemical measurements

A PalmSens 4 electrochemical workstation was used for all measurements. Cyclic voltammograms of 1 mmol/l ferrocene-methanol in 0.1 mol/l KCl solution were recorded to verify the capability of the digital microfluidic electrochemical platform.The scan rate was 50 mV/s.Pb2+both in acetate-buffered solution (pH 5) and in tap water was tested using square wave anodic stripping voltammetry(SWASV).The selected square wave parameters for all measurements were 25 mV amplitude, 20 Hz frequency, and 5 mV step potential. During operation, Pb2+was deposited at −0.8 V, typically for 120 s, and then stripped from−0.6 V to 1.0 V.

III. RESULTS AND DISCUSSION

The digital microfluidic electroanalytical platform is composed of a battery, a control board, an integrated chip, and an electrochemical workstation[Figs.2(a)and 2(b)].The analytical procedure involves transportation of samples to the electrode,electrochemical analysis,and liquid removal for subsequent measurements.

A. Transportation of samples to the electrode

Given the potential applications of our platform in distributed detection systems,a portable control circuit was developed to replace bulky conventional signal generators and amplifiers. As illustrated in Fig.2(c),the power provides a stable 12 V DC voltage to the circuit board.The input voltage is divided into two parts,one of which is converted into a 1 kHz AC signal through the oscillation circuit(DC-AC), while the other is converted into a pair of ±90 V voltages through the boost module (DC-DC). In this way, the source for the on-chip semiconductor relays can be generated by the amplifier.During operation,the programmed signal output from the PC is converted into different pin states by the microcontroller unit(MCU). The states of the relays are correspondingly changed, regulating the movement of the droplet movement to the location of the electrochemical sensor, which is controlled by the workstation connected with the PC.

Figure 2(d) plots the pathway of the droplet before the electroanalytical test. Sample and acetate buffer droplets are loaded at open import zones on the bottom plate. These two droplets are then transported to the mixing zone by continuous application of a high voltage to the electrodes, and they are mixed by back-andforth motion after being merged in the mixing zone. The mixed droplet is then transported to the detection zone, where an additional electrochemical sensor is located on the top plate.Figure 2(e)presents sequential snapshots of one entire detection process. The first three pictures show the process of sample pretreatment along the pathway sketched in Fig.2(d).After detection,the sample needs to be removed for the next measurement. To do this, two 5.5 μl droplets are successively loaded onto the import zone and are transported to the detection zone. The merged droplet (22 μl in total)is then large enough to come into contact with the absorber for removal by capillary force. By repeating the above steps, the same digital microfluidic electroanalytical platform can be used multiple times.

B. Electrochemical characterization on the digital microfluidic electroanalytical platform

1. Sample pretreatment results

The aim of this study is to develop sensing nodes for automatic on-site distributed detection of heavy metals in a water environment (such as a lake). Typically, electrochemical analysis of heavy metals needs to be done in a buffered solution. Thus, sample and buffer should be thoroughly mixed before analysis.However,mixing in channel-based microfluidic electroanalytical platforms normally relies on diffusion, owing to the laminar nature of the flow, or on the use of an extra mixing module,which hampers automation and reduces the efficiency of detection.A digital microfluidic electroanalytical platform has natural advantages in this regard.Sample and buffer droplets are first combined by the electrowetting force, and the combined droplet is then driven to move back and forth along the digital microfluidic patterned electrode to complete the mixing.Highly efficient and reliable mixing by a digital microfluidic chip has been demonstrated in our previous work.29Under the action of the electrowetting force,the entire mixing process only takes 30 s,in contrast to the 6 min required to achieve the same degree of mixing by static diffusion.

2. Condition optimization for Pb2+ test

Pb2+is a common heavy metal pollutant present in water,and in this study it was detected using a gold thin film three-electrode electrochemical cell as a demonstration of the concept of our proposed system. The area of the working electrode has a significant impact on its performance. A larger working electrode area generally gives higher sensor sensitivity. However, to allow complete coverage of the entire three-electrode area by a single transportation of a 11 μl sample,the size of the sensor should not be too large.We prepared four kinds of sensors with different diameters of the working electrode, namely, 0.8, 1.2, 1.4, and 1.8 mm, which corresponded to rectangular working areas(including working,reference,and counter electrodes)of 2.0 mm×1.4 mm,3.6 mm×2.8 mm,4.0 mm × 3.3 mm, and 4.8 mm × 4.0 mm, respectively. We found that the working electrode with a diameter of 1.2 mm provided a balance between analytical performance and the requirements of effective integration.

A cyclic voltametric technique was first employed to evaluate the platform’s workability.Figure 3(a)shows a clear redox response of 1 mmol/l ferrocene-methanol in 0.1 mmol/l KCl aqueous solution transported by the digital microfluidic chip to the electrochemical sensing zone.SWASV was used to sense Pb2+.The effects of accumulation voltage and time were studied in the ranges of −1.0 to−0.6 V and 60 to 360 s, respectively. Depositing at −0.8 V (vs Aupseudo) produced optimized signals. A longer accumulation time in the range studied resulted in a larger stripping current and thus better analytical performance. Nevertheless, the detected droplets are very small in this system and there is a risk of evaporation with time. We chose 120 s as the accumulation time for further experiments.

3. Digital microfluidic electroanalysis of Pb2+in buffer

Figure 3(b)plots stripping voltammograms of Pb2+from 0 to 250 ppb in acetate buffer after deposition at −0.8V for 120 s. The stripping peaks are at around −0.2 V, and the stripping peak current increases with rising heavy metal concentration. The experiments were performed with the same electrodes, although a slight shift in stripping peak potential can be observed that is due to a change of the gold pseudo reference electrode. The extracted peak currents can be fitted linearly in the studied concentration range[Fig.3(c)].The calculated limit of detection(LOD)is 7.6 ppb based on the formula LOD=3×(SD/S),where SD is the standard deviation of three measured peak currents at the minimum concentration(50 ppb)andSis the slope of the calibration curve.It is known that the analytical performance of pure gold thin film electrodes with regard to the detection of heavy metals is not very high. However,there are a variety of electrode modification approaches30-32that can be used to improve it, although these are outside the scope of this study.

To verify the detection accuracy, we also carried out an offplatform electroanalysis of Pb2+(i.e.,without assembly of the electrochemical sensor with the digital microfluidic chip) under the same accumulation and stripping conditions. Figure 3(d) compares the extracted stripping peak currents under on-platform and off-platform conditions. The measurements were done on the same electrode with successive addition of Pb2+solution from low to high concentrations and were then repeated three times under both conditions. The currents for each concentration under on-platform and off-platform conditions are comparable, indicating a negligible influence of digital microfluidic processing.

4. Analysis of real sample (tap water)with the platform

FIG.3.Electrochemical characterization of the digital microfluidic electroanalytical platform.(a)Cyclic voltammogram of 1 mmol/l ferrocene-methanol in 0.1 mol/l KCl solution. (b) and (c) are the square wave anodic stripping voltammograms and the calibrated curve obtained with the extracted peak current of corresponding concentrations of Pb2+. (d)Comparison of on-platform and offplatform stripping peak currents of Pb2+.Each point represents the mean value of three replicates. The error bars are the corresponding standard deviations.

FIG. 4. Electrochemical study of Pb2+in tap water using the digital microfluidic electroanalytical platform. (a) Stripping voltammograms and (b) extracted peak current for concentrations of dopant Pb2+ from 0 to 200 ppb.The error bars are the corresponding standard deviations of three replicates.

To test the applicability of the digital microfluidic electroanalytical platform, the Pb level in tap water from Tianjin University was analyzed.An ICP-MS measurement was performed,and almost no Pb2+was found in our collected tap water (the given value of 0.069 ppb is beyond the accuracy of the ICP-MS equipment).

We doped certain amounts of Pb2+(50, 100, 150, and 200 ppb) in the tap water and analyzed them electrochemically with the digital microfluidic electroanalytical platform.For measurement,we dropped tap water and buffer onto the platform. After mixing, they were analyzed by the electrochemical sensor. Figure 4(a)shows the corresponding stripping responses, and it can be seen that no Pb2+signal appears in the undoped tap water, whereas clear increasing peaks are found with increasing amount of Pb2+dopant. Figure 4(b) plots the extracted peak current values. Each point is the mean of three measurements, together with the corresponding standard deviation. The peak current at each dopant concentration was extracted and interpolated in the calibration curve [Fig. 3(c)]. Each concentration was measured three times.Table I lists the doped and sensed concentrations. Good sensing accuracy is obtained, with recovery between 84.6% and 101.3%.These initial results indicate the feasibility and practicality of the digital microfluidic electroanalytical platform for heavy metal analysis.

Although our platform has been tested here for the detection of Pb2+, it can be applied to the detection of many other hazardous substances (e.g., cadmium ions, mercury ions, and pesticides) by modifying or replacing the electrochemical sensors. For instance,bismuth-based electrodes can be used for the sensing of lead and cadmium ions,and gold electrodes for mercury ions. Carbon electrodes can provide the base for many sensing applications. Moreover, modifying these electrodes with nanomaterials can improve their sensitivity and may also broaden the category of detectable substances.33Therefore,this platform shows great potential for on-line water quality monitoring applications.

TABLE I.Comparison of doped and measured Pb2+concentrations in tap water.

C. Power consumption of the platform

Our aim is the continuous and automatic detection of heavy metals in the field as an Internet-of-Things application.Low power consumption is an essential requirement for wide deployment of a large number of sensing nodes in a distributed manner, as well as for long-term use.In this study,we calculate the power consumption of the digital microfluidic electroanalytical platform as the applied voltage multiplied by the flowing current.The total power consumption includes that needed for electrochemical detection and that needed for liquid movement by the digital microfluidic chip, and the latter is dominant as the former consumes negligible power in comparison.

Here,the battery capacity is 3500 mA⋅h and it supplies a voltage of 12 V. The total working current of the digital microfluidic chip is 21 mA. Therefore, the total power (the product of working current and voltage) of the system is 252 mW. The detection node can work continuously for 166 h. Moreover, for most practical applications, a single detection per day could be sufficient, and the detection should not take longer than 20 min (three replicates). In this case, the current system can work for more than one year without the battery needing to be changed. Compared with commercial water quality testing instruments (where,for instance, the working voltage of a Hach EZ Metal Analyzer is 110-240 V,and the working current is 4 A),the platform presented here has obvious advantages of miniaturization and low power consumption.

It is worth noting that the power consumption of the system can be considerably reduced. For example, by changing the dielectric layer and structure of the digital microfluidic electroanalytical platform, the droplet driving voltage could be reduced to 2 V.34Correspondingly, the control circuit of the sensing node does not require a boost circuit module (190 mW in this work),which significantly reduces its power consumption. With further improvement, it is expected that the overall power consumption of the system could be optimized to the level of few milliwatts. Thus, the system could last for more than 10 years without intervention.

Apart from low power consumption, miniature size and intelligent behavior are two more key requirements for sensing nodes. As presented in Fig. 2(b), the present platform has dimensions of 32 cm × 22 cm. In our next step in the development of the system, the path of droplet movement will be preset by the MCU rather than by the PC. With the use of a specially designed miniature electroanalytical board, the entire system can be expected to be reduced to be less than palmsized. In addition, more advanced functions such as wireless data transmission and sensor hubs with multiple electrochemical sensors (microwatts per sensor) could be integrated into the system, and, after appropriate encapsulation and with suitable fixings, these could be distributed in different parts of a water environment to continuously monitor heavy metal pollutants.

IV. CONCLUSIONS

We have developed a proof-of-concept distributed sensing node for detecting heavy metal ions in water, featuring low power consumption and compact size.The sensing node consists of a digital microfluidic electroanalytical chip,an electrical circuit to manipulate liquids on the chip, and an electrochemical workstation to record electrochemical responses. Assembling an extra gold thin film electrochemical sensor chip and a liquid absorber on the ground electrode of the digital microfluidic chip enables the platform to perform continuous multiple detections. Application of the platform to accurate Pb2+detection both in buffer solution and in tap water demonstrates its feasibility and applicability for heavy metal analysis.The power consumption is quite low,and the sensing node may work for several years with daily measurement being powered by a normal battery. Further development and optimization of the system will focus on reducing its size and power consumption and on diversification of sensors,which will facilitate a wide range of applications in distributed detection nodes for determination of water quality.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (Nos. 51375341, 61901295, and 62001322)and the Nanchang Institute for Microtechnology of Tianjin University.

The authors declare that they have no known competing financial interests or personal relationships that could appear to have influenced the work reported in this paper.