# Electrochemical Performance of Micropillar Array Electrodes in Microflows

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Methods

#### 2.1. Materials and Instrumentations

_{3}[Fe(CN)

_{6}]), potassium ferrocyanide (K

_{4}[Fe(CN)

_{6}]), potassium chloride (KCl) and 1H,1H,2H,2H-perfluorooctyltrichlorosilane were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Ultrapure water (18.2 MΩ∙cm

^{−1}) was used for dilution and all experiments were carried out at room temperature. All chemicals were analytical grade and used without further purifications.

#### 2.2. Configuration of Microchip-Based Electrochemical Detection System (μEDS)

^{2}planar area. The μAEs with varying micropillar heights (100, 200 and 300 μm) and spacing between two single micropillars (150, 200 and 250 μm) were modeled and investigated numerically, of which the specific parameters are listed in Table 1.

#### 2.3. Numerical Simulation Method of the μEDS

#### 2.3.1. Theory

_{o}(t) and C

_{R}(t) are the concentration of the analyte at the electrode surface at time t. k

_{f}and k

_{b}are the forward and reverse reaction rate constants, which can be expressed as:

_{0}is the standard heterogeneous rate constant; α is the transfer coefficient; R is the gas constant; T is the absolute temperature; E is the potential applied to the electrode; E

^{0′}is the equilibrium potential.

#### 2.3.2. Numerical Model

#### 2.4. Fabrication of μEDS

#### 2.5. Experiments of the Electrochemical Detection

_{3}[Fe(CN)

_{6}]/K

_{4}[Fe(CN)

_{6}] with 0.1 M KCl were used in all experiments of this research. The CV experiments were firstly performed with the flow rate of zero, the scan rate of 0.05 V/s and the voltage range of −0.2 to 0.6 V. Then the potential corresponding with the peak current of the cyclic voltammogram was applied to the working electrode to perform the CA experiments, in which the flow rates in the microchannel varied from 0 to 30 μL/min. Finally, the steady-state response current of the CA experiments was recorded for the further analysis.

## 3. Results and Discussion

#### 3.1. Effect of Flow Rate and Spacing

^{2}), therefore the number of micropillars and the total surface area of μAE are confined by the spacing between two adjacent micropillars. The effect of the spacing on the current response was analyzed. Besides the height of micropillar, which was set as a constant 300 μm, the other parameters of μAEs used for numerical study were the same as listed in Table 1, and as a reference, the planar electrode was also considered.

_{d}) were defined and calculated, as shown in Figure 5d. Obvious tail effect, which is characterized by the large current density ratio, is usually existed in cases where the spacings of μAEs are small (150 or 200 µm for instance) or the flow rates are relatively low (<10 µL/min). In such circumstances, as shown in Figures S2a and S3a, there is a wide range of low concentration downstream, and the concentrations detected by the front and back micropillars, as well as the corresponding current responses, differ greatly. This feature restricts the benefits of the μAE brought by the increased reaction area, and further increment of the micropillars number doesn’t lead to improved detection performance.

#### 3.2. Effect of Micropillar Height

#### 3.3. Effect of Micropillar Layout

#### 3.4. Effect of Micropillar Shape

#### 3.5. Experimental Verification

## 4. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**The schematic diagram of (

**a**) the μEDS; (

**b**) the μAE. (

**c**) Blow-up view of the μAE with the definition of the geometrical parameters.

**Figure 2.**(

**a**) The computational domain of the μEDS; (

**b**) The schematic diagram of the meshing method.

**Figure 3.**The fabrication process of the μEDS. (

**a**,

**b**): fabrication of the negative PDMS masters of μAE and microchannel; (

**c**,

**d**): fabrication of the μAE and microchannel; (

**e**): deposition of the conducting layer; (

**f**,

**g**): oxygen plasma treatment; (

**h**): integration of the detection microchip.

**Figure 4.**The SEM images of fabricated micropillars. (

**a**) top view of micropillars; (

**b**) side view of micropillars.

**Figure 5.**(

**a**) Current responses of μAEs with various spacings at different flow rates and spacings; (

**b**) Current responses of μAEs with various surface area at different flow rates; (

**c**) Current and area ratios of μAEs with various spacings at different flow rates; (

**d**) Ratios of the current density between the first and last row of micropillars.

**Figure 6.**(

**a**) Current responses of the planar electrode and the μAEs with micropillars of different heights under varying flow rates; (

**b**) The current density ratios of the μAEs with micropillars of different heights under varying flow rates.

**Figure 7.**(

**a**) Current responses of the planar electrode and the μAEs in different layouts under varying flow rates; (

**b**) The current density ratios of the μAEs in different layouts under varying flow rates.

**Figure 8.**(

**a**) Current responses of the planar electrode and the μAEs with micropillars in different shapes under varying flow rates; (

**b**) The current density ration of the μAEs with micropillars in different shapes under varying flow rates.

**Figure 9.**(

**a**) Experimental CV of the planar microelectrode and μAE200 at the scan rate of 0.05 V/s; (

**b**) Experimental and simulated current response of the planar microelectrode and μAE200 at different flow rates; In the 5 mM K

_{3}[Fe(CN)

_{6}]/K

_{4}[Fe(CN)

_{6}] solutions with 0.1 M KCl vs. Ag/AgCl.

Parameters | Planar | Conical Micropillar | ||
---|---|---|---|---|

Projection area l × w (mm^{2}) | 1.5 × 2.5 | |||

Top radius r_{t} (μm) | - | 25 | ||

Base radius (μm) | - | 50 | ||

Height h (μm) | - | 100/200/300 | ||

Spacing d (μm) ^{1} | - | 150 | 200 | 250 |

Number of pillars n | - | 136 | 78 | 55 |

Surface area S (mm^{2}) | 3.75 | 7.33 | 8.82 | 12.60 |

Area ratio ^{2} S_{g} | 1.0 | 1.95 | 2.35 | 3.36 |

^{1}Spacing between the centers of two adjacent micropillars.

^{2}The ratio of the active area between the μAE and the planar electrode.

Parameters | Unit | Value |
---|---|---|

Diffusion coefficient D | m^{2}/s | 6.5 × 10^{−5} |

Faraday’s constant F | C/mol | 96,485.33 |

Standard heterogeneous rate constant k_{0} | m/s | 1 × 10^{−4} |

Transfer coefficient α | - | 0.6 |

Gas constant R | J/(mol·K) | 8.314 |

Absolute temperature T | K | 298.15 |

Applied potential E | V | 0.25 |

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## Share and Cite

**MDPI and ACS Style**

Liu, B.; Lv, C.; Chen, C.; Ran, B.; Lan, M.; Chen, H.; Zhu, Y.
Electrochemical Performance of Micropillar Array Electrodes in Microflows. *Micromachines* **2020**, *11*, 858.
https://doi.org/10.3390/mi11090858

**AMA Style**

Liu B, Lv C, Chen C, Ran B, Lan M, Chen H, Zhu Y.
Electrochemical Performance of Micropillar Array Electrodes in Microflows. *Micromachines*. 2020; 11(9):858.
https://doi.org/10.3390/mi11090858

**Chicago/Turabian Style**

Liu, Bo, Chuanwen Lv, Chaozhan Chen, Bin Ran, Minbo Lan, Huaying Chen, and Yonggang Zhu.
2020. "Electrochemical Performance of Micropillar Array Electrodes in Microflows" *Micromachines* 11, no. 9: 858.
https://doi.org/10.3390/mi11090858