ON-DEMAND CONTROL OF PH IN MICROFLUIDIC DROPLETS
H.B. Zhou1,2 and S.H. Yao1*
1
Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Hong Kong,
CHINA and
2
State Key Laboratory of Transducer Technology, Shanghai Institute of Microsystem and Information
Technology, Chinese Academy of Science, CHINA
ABSTRACT
In this paper, we propose a strategy to form on-demand droplets with specific pH values. We analyzed the principle
of our technique, designed and fabricated the microfluidic devices. The droplet-on-demand system is based on a Tjunction microchannel for droplet emulsion and a pulsed pressure to trigger the onset of droplet injection. The pH
control is based on electrolysis of water in the microchannel, and the produced hydrogen and hydroxyl ions are separated
and confined in individual containers during the droplet generation. By tuning the applied voltages and pressure pulses,
we can on-demand control the pH value in a droplet.
KEYWORDS: pH value, droplet, microfluidics, on-demand
INTRODUCTION
Control of pH in microscale environments offers many intriguing possibilities for biological applications such as onchip proteolysis, protein crystallization, cell or biomolecular separation [1]. Conventional techniques (such as carbon
dioxide dissolution, regulation of acid and base solutions, and so on) have been developed for pH modulation on a large
scale, but are difficult for miniaturization and integrated into microfluidics. Using electricity to split or electrolyze water
for pH control is one promising method for its easy manipulation and good compatibility with microfluidics. However,
for the water splitting technique, complex functional modules (e.g., bipolar membranes or nanochannels) are essential to
separate the H+ and OH- ions into two parts [1, 2]. For water electrolysis, H+ and OH- ions are produced near the anode
and cathode, respectively [3]. Meanwhile, the concomitant products (H2 and O2) may form gas bubbles and disturb the
flow in microfluidics. Moreover, the generated solution in water splitting or electrolysis is in the continuous phase [1-3];
so the diffusion effect will hinder the formation of a precise and stable and pH environment. In order to maintain the pH
value in the vicinity, an electric voltage need to be applied continuously. To overcome these drawbacks, we propose ondemand microfluidic droplet techniques to form isolate compartment with pH regulation.
THEORY
The basic principle of our method is water electrolysis. When a voltage is applied to a pair of electrodes, the water is
electrolyzed. H+ and OH- ions are generated and accumulated near the anode and cathode (Fig. 1a,c), according to[4]
Anode:
(1)
2 H 2O = 4 H + + 4e + O2 ↑
Cathode:
4 H 2O + 4e = 4OH − + 2 H 2 ↑
(2)
Fig 1: pH control by electrolysis of water. (a) The electrolysis and separation process. (b) pH change in solutions near
the electrodes across a microchannel. In the experiments, hydrion pH indicator solution (UI-100) was used.
According to the law of charge conservation, the production (Q) is decided by Q=I*t, where I is the current in the
electrolysis reaction, and t is the electrolysis time. By tuning the current I and time t, we can control the production of the
H+ and OH- ions. However, when the voltage is removed, the H+ and OH- ions diffuse and neutralization happens.
Therefore, if the H+ and OH- ions are separated and confined in individual containers, solutions with stable pH will be
obtained (Fig. 1b).
978-0-9798064-6-9/µTAS 2013/$20©13CBMS-0001
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17th International Conference on Miniaturized
Systems for Chemistry and Life Sciences
27-31 October 2013, Freiburg, Germany
EXPERIMENTAL
Design of the microchip
Fig.2 shows the schematic of the microchip. To facilitate on-demand control of droplets, we introduce an injection
channel with a high flow-resistant, a nozzle and a T-junction in the microchip [5]. By tuning the pulsed pressure, the
formation of nanoliter droplets can be controlled in time and volume [5]. To electrolyze water in the microchannel, we
integrate a pair of electrodes into the injection channel. As illustrated in Fig. 2, one electrode is integrated at the end of
the injection channel while the other electrode is located near the nozzle.
At the beginning, pressure Pm and Pi is applied to the oil and water inlets to maintain a water/oil interface at the
nozzle. In each cycle, a voltage pulse is firstly applied; H+ or OH- ions are generated and accumulate near the nozzle. By
tuning amplitude and duration of the voltage pulse, we can precisely tune the amount of the produced ions. Next, a
pressure pulse is applied to the injection channel to inject the solution near the T-junction from the nozzle and form a
droplet. In this way, a droplet with a specific pH value is formed.
Fig 2: Schematic illustration of the microchip. The inset is a magnified image of the nozzle at the T-junction.
Fabrication and setup
The device consists of a poly(dimethylsiloxane) (PDMS) layer containing microfluidic channels and an electrodepatterned quartz substrate. The microchannels were fabricated in PDMS using a standard soft lithography procedure. A
negative photoresist (SU-82050, MicroChem) was spin-coated and patterned on a silicon wafer. PDMS mixture in a 10 :
1 ratio of prepolymer and curing agent (Sylgard 184, Dow Corning) was degassed and poured over the mold and cured
in an 80 ℃ oven for an hour. The cast PDMS was then peeled off from the mold and inlet and outlet holes were punched
using a pan head needle on the PDMS replica. To make the microelectrodes, an adhesive layer of Ti/W (200 Å) and
2000 Å of platinum were sputtered on a photoresist patterned quartz wafer, and a lift-off process was applied to form the
electrode patterns. Then an insulation layer of silicon oxide with a thickness of 1 μm was deposited on the patterned
quartz substrate using PECVD, and finally the contacting pads for electrical connection were exposed. The PDMS
replica and quartz substrate were cleaned, treated with oxygen plasma, aligned under the microscope and then bonded
together to seal the microchannels. The bonded device was heated at 105℃ on a hotplate for 24 h to increase bonding
strength and recover the hydrophobicity of the PDMS-based microchannels.
Constant pressure was applied to each phase using a compressed air system regulated by pressure transducers
(2KSNNF01, Marsh Bellofram). A solenoid valve (35A-ACA-DDAA, MAC valves, INC.) was used to impose a
pressure pulse on the water phase. To increase the conductivity of water phase, 0.1 M Na2SO4 was used. Hydrion pH
indicator solution (UI-100, Micro Essential Laboratory Inc.) was chosen for its wide range.
RESULTS AND DISCUSSION
The fabricated microchips are illustrated in Fig. 3a. In the experiments, voltage pulses with a duration of 100 ms and
amplitude stepping from -5V to +5V were applied to the electrodes. Pressures of 1.8 psi and 1.9 psi were applied to the oil
and water phases to maintain a water/oil interface at the nozzle. A pressure pulse with a duration of 70 ms and amplitude
of 1 psi was applied to the water phase to form a droplet of ~1 nL. Fig. 3c show the results for the electric pulse stepping
from -5V to +5V. Droplets with a volume of ~1nL were formed while their pH values varied from 11 to 3. The pH values
were estimated by mapping the color of the solutions with the color map. For an applied voltage of 5 V with a duration of
100 ms, a current of ~1 μA was measured through the system, and the total transferred electrons were estimated as 1.6E12.
According to the electrolysis reaction (Eqs. 1-2) , the same amount of OH- and H+ ions was produced, which results in a pH
value of ~3 or ~11 in 1 nL solution. Meanwhile, the production of the O2 and H2 gas were estimated as 10 pL and 20 pL,
respectively, which were two orders lower than the droplet volume. We expect the gas generation in our system may
dissolve in the solution, so no bubbles were detected in the microchannels (in Fig. 3c).
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In the experiments, the interval between the electric pulse and the pressure pulse was 100 ms. According to the diffusion
equation, the diffusion length L is expressed as: L = Dt , where t is the diffusion time and the D represents the diffusion
coefficient (DH+=9.3E-9 m2/s, DOH-=5.3E-9 m2/s). Hence the diffusion length is about 25 μm which is relatively smaller than
the length of droplet (~300 μm in the experiments) in the injection channel. Importantly, after the droplet being formed, the
diffusion is restricted in the isolated droplet. Therefore, the pH value in the droplet remains stable. Fig. 3b show droplets
with pH ranging from 3 to 11 trapped in a chamber array. No obvious changes were found even after several hours.
Fig 3: (a) The fabricated microchips. (b) Trapped droplets with pH values from 3 to 11. (c) Snapshots of the formation of pH-adjustable droplets by stepping the voltage pulses from -5V to +5V. In the experiments, Hydrion pH indicator solution (UI-100) was used.
CONCLUSION
In this paper, we present a simple and adaptive microfluidic system that enables on-demand formation of droplets
with pH regulation in an isolated compartment. By tuning the applied voltage and pressure pulses, the pH value in a
droplet can be well controlled. Compared with other methods, our methods is simple in fabrication and flexible in
operation. We believe our on-demand control of pH value in droplets will find important applications in the fields such
as biological assay, protein crystallization, enzyme assay, and so on.
ACKNOWLEDGEMENTS
This work was supported by the Direct Allocation Grant (No. DAG12EG07-13) from HKUST and the National Science Foundation of China (No. 61006086). The authors would thank Dr. Gang Li in SIMIT for his useful suggestions..
REFERENCES
[1] L. J. Cheng and H. C. Chang, "Microscale pH regulation by splitting water," Biomicrofluidics, 5(4): 046502, (2011).
[2] E. O. Gabrielsson, K. Tybrandt, et al., "Ion diode logics for pH control," Lab on a Chip 12(14): 2507-2513 (2012).
[3] K. Macounova, C. R. Cabrera, et al., "Generation of natural pH gradients in microfluidic channels for use in isoelectric focusing," Analytical chemistry 72(16): 3745-3751(2000).
[4] A. J. Bard, L.R. Faulkner, " Electrochemical Methods: Fundamentals and Applications", chapter 1, (2001)
[5] H. B. Zhou and S. H. Yao, "A facile on-demand droplet microfluidic system for lab-on-a-chip applications ", microfluidics and nanofluidics, Accepted.
CONTACT
*S.H. Yao, tel: +852 23587205; [email protected].
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