High resolution microfabricated gas chromatography column with porous silicon acting as support

Accepted Manuscript Title: High Resolution Microfabricated Gas Chromatography Column with Porous Silicon Acting as Support Author: Jianhai Sun Dafu Cui Fengying Guan Xing Chen Lulu Zhang PII: DOI: Reference:

S0925-4005(14)00487-0 http://dx.doi.org/doi:10.1016/j.snb.2014.04.076 SNB 16842

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

4-1-2014 21-4-2014 23-4-2014

Please cite this article as: J. Sun, D. Cui, F. Guan, X. Chen, L. Zhang, High Resolution Microfabricated Gas Chromatography Column with Porous Silicon Acting as Support, Sensors and Actuators B: Chemical (2014), http://dx.doi.org/10.1016/j.snb.2014.04.076 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High Resolution Microfabricated Gas Chromatography Column with Porous Silicon Acting as Support Jianhai Suna,*, Dafu Cuia, Fengying Guan b,*, Xing Chena, and Lulu Zhanga a

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State key laboratory of transducer technology, Institute of Electronics, Chinese Academy of Sciences

，Beijing 100190, China

International Centre for Bamboo and Rattan, Key Laboratory of Bamboo and Rattan, Beijing 100102,

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b

China

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[email protected]

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*Corresponding author: Tel: +86-10-58887183; +86-10-84789808; E-mail:[email protected];

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Abstract: In this paper, a micro gas chromatography (GC) column with embedded micro-pillars was developed for gas chromatography. These micro-pillars positioned in the channel can reduce effective width and increase overall surface area of column. Moreover, a

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layer of porous silicon acting as support was produced on the channels and micro-pillars wall, which are able to coat more stationary phase and increase its

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thickness, leading to higher separation resolution.Therefore, the proposed column had higher sample capacity as the overall surface area of the channels was much larger

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than that of the column without porous silicon. The results indicated that the proposed column separated a environmental sample mixture of 6 compounds within 65 seconds,

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achieved a high resolution with these adjacent chromatography peaks separated completely and a separation efficiency of 10,000 plates/m, and eluted highly

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symmetrical peaks.

Keywords: Micro Gas Chromatography Column; Porous Silicon; Separation

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Efficiency; Overall Surface Area

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1. Introduction: Micro gas chromatography (GC) column, due to its small size, high separation efficiency and capability of being easily integrated, was used to develop into a micro chromatography system [1, 2], which was very suitable for field use, such as indoor

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and ambient air quality, climate change, industrial emissions control, breath analysis and so on. Therefore, this research direction is under intensive research due to its

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potential application in the area of field-portable instrumentations.

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Numerous studies have focused on working on the development of miniaturized GC columns using MEMS technology that can quickly separate samples with high efficiency [3–6]. These micro GC columns have advantages, such as small size, fast

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response, low power consumption and batch production. Therefore, these attributes make these devices attractive for a number of applications involving on-site

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monitoring of environmental samples.

Many efforts to improve separation efficiency have been made in many works, and

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these efforts included use of effective stationary phase on the silicon column walls [7,

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8], fabrication of high aspect ratio of the channels [5], use of static coating method for thick and uniform stationary phase film [9], improvement of sample capacity by

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fabricating a multicapillary column [10] and fabrication of micro columns with embedded micro-pillars which are able to coat more stationary phase and reduce effective width of the column [11, 12]. Although these efforts have yielded important progress, stationary phase thickness (ranged from 0.1 μm to 0.2 μm) and sample capacity of the columns were limited, which resulted in a relatively low resolving power.

In this paper, In order to increase sample capacity of micro column, a micro GC column with embedded micro-pillars was developed for gas chromatography. These micro-pillars positioned in the channel can reduce effective width and increase overall surface area of the column. Moreover, a layer of porous silicon acting as support was produced on the surface of the channels and micro-pillars (refer to Fig.1), which is

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able to coat more stationary phase and increase its thickness, leading to higher separation resolution. 2. Experimental Section

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2.1. Fabrication of the micro GC column

In the following, the details of the fabrication process of the micro GC column were

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reported: Firstly, a layer of 300 nm silicon nitride was deposited by a low pressure chemical vapor deposition technology on a (100) n-type low-resistivity silicon wafer

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(low resistivity of the silicon wafer enables a good contact between the wafer and its backside metallic connection) with a thickness of 550 μm and a diameter of 76 mm,

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and then a layer of 200 nm aluminum film was deposited by an electron-beam evaporation technology on top of silicon nitride layer, served as a mask for silicon

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etching. Secondly, a layer of 2 µm photoresist was coated on the wafer and patterned as a mask for aluminum etching. Subsequently, aluminum without protection of the photoresist was etched away by a H3PO4 etching agent with silicon nitride surface

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exposed. Then, a reactive-ion etching technology was used to etch the silicon nitride

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and a deep reactive-ion etching was conducted to form rectangular micro channels and micro-pillars. The diameter of micro-pillars is 50 µm, the length, depth and width

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of the micro channels was 2 meter, 350 µm and 320 µm, respectively (refer to Fig.2). Therefore, aspect ratio of the channels was increased from 1.2:1 to 3:1 due to the embedded micro-pillars, and overall surface area of the proposed column was 5 times more than that of the column without these micro-pillars. Then, a layer of porous silicon was produced on the surface of the micro channels and micro-pillars. In this paper, porous silicon was fabricated by electrochemical anodization, and the details of the fabrication process were reported as follows. Firstly, electrolyte used for the fabrication of the porous silicon was composed of purity hydrofluoric acid (HF) in 40% aqueous solution diluted in ethanol. This concentration [13-15] was considered to give a large porosity variation, and ethanol was often added at the mass ratio of 1:1, to facilitate evacuation of H2 bubbles which developed during the process. It is well known that during the reaction there was a hydrogen release and 4

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bubbles were formed on the surface of the silicon wafer. A current applied during the etching was provided by a current source. A typical setup of fabrication of porous silicon is schematically shown in Fig. 3. A layer of porous silicon with a thickness of 4 μm (refer to Fig.4) was fabricated on the surface of channels and micro-pillars by

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applying a current density of J=50 mA/cm2 for 30 minutes which allows a better control of porosity, thickness and reproducibility of the porous silicon. The pores

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radius measured by AFM (Atomic Force Micro scope) ranged from 5 to 50 nm, and the porosity can be calculated by the the volume ratio: Vp

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ε=

(1)

VSi

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Where V p is the pore volume of the porous silicon, and VSi is the total volume of the porous silicon. As the density of pore is consistent with porous silicon, the porosity

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can also be expressed as equation (2) when the volume ratio is converted to mass ratio.

(2)

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ε = (m1 − m2 ) (m − m ) 1 3

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Where m1 is the mass of column without porous silicon, m2 is the mass of column

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with a layer of porous silicon, and m3 is the mass of column which the porous silicon was etched away by 1% KOH etching agent. Aiming to measure the porosity, a micro column was fabricated by the same procedure as the proposed column. By weighing the column in different states, the masses of them ( m1 , m2 and m3 ) are 4.6761 grams,

4.5147 grams and 4.4743 grams, respectively, and then the calculated porosity is nearly 80% from the equation (2). The porous silicon with a large surface area was acting as support which is able to coat more stationary phase and increase its effective thickness.

2.2. Fabrication of the micro heaters In order to reduce volume of the GC system and rapidly evaluate temperature of the column, micro heaters and temperature sensors (T-sensors) were fabricated on the backside of the micro GC column. These heaters and T-sensors were fabricated as a 5

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20 nm/250 nm Cr/Pt stack deposited by a magnetron sputtering technology and patterned by a lift-off technology. The resistances of the heaters and temperature sensors were all 8 Ω and 100 Ω, respectively. The heaters and T-sensors enable the temperature of the micro GC column to be increased at a speed of 20°C per 10

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seconds and the temperature could be raised up to 200 °C in less than 2 minutes. Then, a Pyrex 7740 glass wafer with a thickness of 1 mm and a diameter of 76 mm was

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bonded to the silicon wafer. Fig.5 shows the heaters and the fabricated micro GC column.

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2.3. Stationary Phase Coating

In order to produce a thick and uniform stationary phase film on the channels and

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micro-pillars, the fabricated column was coated with stationary solution via a dynamic coating procedure. The dynamic coating is defined that a small amount of stationary

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phase solvent is pushed into the micro GC column, and flows through the column with help of a non-reactive gas, and then excess solvent is removed by continuous gas

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flow. The thickness of the coating film depends on velocity and concentration

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stationary phase solvent. The details of coating process of the micro GC column were reported as follows. Firstly, a solution of SE-54 with a mass of 5 mg mixed with a

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solution PEG-20M with a mass of 5 mg was dissolved in CHCl3 solvent and used to fill the column with the coating solution concentration of 10 mg/ml. Then, one end of the column was connected with a capillary which was emerged into the coating solution, the other end of the column was connected with a laboratory made micro-pump, and the column was kept in a water bath at 40°C. After the column was full of the coating solution, nitrogen gas was delivered through the column for a few hours to completely evaporate the CHCl3 solvent. Subsequently, the column was put into a temperature programmed oven under a nitrogen flow inside. The temperature of the oven was firstly increased gradually by 5 °C/min until 120°C, and then the temperature of the oven was kept at 120°C for 4 hours. Finally, the proposed column (the column with embedded micro-pillars & porous silicon & stationary phase acted as the proposed column) was heated at 200°C for 4 hours to cure the stationary phase

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to slow evaporation of the solvent, and the amount of active phase is greatly increased with respect to a reference column (the column with embedded micro-pillars & stationary phase acted as the reference column) with a stationary phase thickness of 0.15 μm, resulting in a 4 μm thick porous silicon layer completely coated by the

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stationary phase.

2.4. Apparatus and equipment

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In order to evaluate capabilities of the proposed column, separation experiments were

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conducted using a laboratory made portable photoionization detector (PID). The pure He was used as carrier gas and gas mixture (provided by the Beijing Hua Yuan Gas Chemical Industry Co., Ltd) was composed of 6 components (benzene, toluene,

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ethylbenzene, styrene, nonane and decane) with the same concentration of 5 ppm. The sample was injected by a micro pump and a 6-port external sample injector, and

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carrier gas velocity was controlled by a gas flow controller.

3.1. Column Efficiency

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3. Results

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The GC column can be divided into a number of theoretical plates, where, a plate is

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the smallest section of the column where an analyte can achieve equilibrium between the two phases. The length of each plate is termed as height equivalent to a theoretical plate (HETP), and the HETP can be expressed as Equation (3) [16].

HETP = 2

2 1 + 9k + 25.5k 2 w2 f1 2kd f ( w + h) f1 f 2 + ( )u + 105(1 + k ) 2 Dg f 2 3Ds (1 + k ) 2 h 2 u

Dg

(3)

where Dg and Ds are binary diffusion coefficients in mobile and stationary phases, respectively, df is the stationary phase thickness, w and h are the channel width and

height, respectively, and f1 (varies between 1 and 1.125) and f2 (varies between 0 and 1) are the Gidding-Golay and Martin-James gas compression coefficients, respectively. The stationary phase thickness, df, of the proposed column and the reference column are 4 μm and 0.15 μm, respectively. A k value of 2 and an h value of 350 μm and an 7

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effective width w value of 120 μm was used in the calculations. The trend line in Fig.6 shows calculated height equivalent to a theoretical plate versus average carrier gas velocity from equation (3). As we can see from the Fig.6, the minimum HETP value, Hmin, found at the optimal average carrier gas velocity, uopt, gives the maximum

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number of theoretical plates. For the proposed column a minimum HETP of 0.0093

cm (10,700 plates/m) at a linear gas velocity of 18 cm/s can be expected from the

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calculations, however, for the reference column a minimum HETP of 0.0091 cm (11,000 plates/m) at a linear gas velocity of 18 cm/s can be expected from the

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calculations.

Based on above comparison of the performance, both the proposed column and the

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reference column can achieve high column efficiency. Although, an increase of the thickness of stationary phase film causes a slightly decrease in the separation

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efficiency, the proposed column can achieve a higher resolution due to its larger surface area of the column channels and thicker stationary phase. The resolution is a

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drawback for the micro column due to its limited length, because the short column lengths associated with microfabricated GC systems can reduce chromatographic

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resolution. Therefore, increasing the resolving power is an important step for

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configuring the micro GC column. 3.2. Separation Chromatograms

In order to evaluate the capability of the proposed column, the separation experiments were performed under isothermal conditions at 40 °C with a carrier gas linear velocity of 18 cm/s (or the volume flow rate of 1.2 sccm) and the reference column was used as a control. The chromatograms (refer to Fig.7) were acquired with an injection volume of 350 µl, Fig.7. (a) shows the chromatogram of the proposed column, and Fig.7. (b) indicates the chromatogram of the reference column. As we can see from these results, the proposed column separated gas mixtures in less than 65 seconds. The plates (n) can be calculated based on the retention time ( t R ) of the peak [17].

n = 5.54(

tR 2 ) w1 / 2

(4)

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Where t R is the retention time, and w1 / 2 is the width of the peak at half height. The proposed column yielded an efficiency of 10,000 plates/m, which was close to the theoretical fit. Moreover, separations were performed for three times at the same

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conditions to verify the repeatability of the proposed column, and Fig.8 indicated that the proposed column demonstrated a good consistency in column efficiency. In

addition, resolution, which is used to express the degree of separation between two

tr2 − tr1 1 ( w1 + w2 ) 2

=

2(tr2 − tr1 )

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R=

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adjacent chromatography peaks, is defined by [3]:

w1 + w2

(5)

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The subscripts 1 and 2 denote the first and second peak, respectively, and w is the width of the peak at the base. The calculated resolution of the proposed column

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between these adjacent chromatography peaks were all over 1.5, which denote these components could be completely separated.

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However, the reference column rapidly separated the gas mixtures in less than 55

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seconds, and yielded an efficiency of 11,000 plates/m at a carrier gas linear velocity of 18 cm/s. The results indicated that the retention time was increased and the column

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efficiency was slightly decreased with the increase of the stationary phase thickness, which was consistent with the theoretical analysis. However, the resolution between benzene and toluene, nonane and decane were 1.4 and 1.35, respectively, which indicated these chromatography peaks could not be completely separated. In addition, a comparison of the tailing factors (which can be used to quantify the peak tailing) of the two columns is shown in Table 1. As we can see from the peak tailing of the proposed column, the calculated tailing factors of the six chromatographic peaks were all less than 1.10 which demonstrated the gas chromatography peaked symmetrically. However, the calculated tailing factors of the six chromatographic peaks of the reference column were larger than that of the proposed column, and the calculated tailing factors of benzene, toluene, nonane and decane were 1.13, 1.16, 1.11 and 1.14, respectively, which obviously show peak 9

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tailing (refer to the zoomed peaks of benzene, toluene, nonane and decane in Fig.7. (b)). Thus, the proposed column achieved a higher resolution and a higher sample capacity

which is very important for separating complicated compounds.

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4. Discussion

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by fabricating a layer of porous silicon on the surface of channels and micro-pillars,

The work here demonstrates that it is possible to fabricate a micro GC column with a

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layer of porous silicon on the micro-pillars and channels that can efficiently separate environmental gas mixtures. Based on above experimental results, the fabricated

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column achieved high column efficiency due to its larger surface area. Thus, the porous silicon fabricated on the micro-pillars and channels was acted as support,

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which dramatically increased the sample capacity and led to a higher separation resolution. Because the performance (such as the porosity, the thickness, and the radius of pores) of the porous silicon could directly affect the overall surface area,

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therefore, in this paper, an optimal method was used to fabricate the porous silicon

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based on previous works. However, HETP slightly decreased with the increase of the stationary phase thickness, therefore, equilibrium should be achieved by choosing the

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thickness of the porous silicon. 5. Conclusions

In this paper, a layer of porous silicon with a thickness of 4 μm acting as support which is able to coat more stationary phase and increase its thickness was successfully fabricated on the surface of the channels and micro-pillars. Through the above experimental tests, the results indicated that the proposed column yielded an efficiency of 10,000 plates/m, leading to a high separation efficiency. Moreover, the calculated resolution of the proposed column between these adjacent chromatography peaks were larger than that of the reference column, moreover, the calculated tailing factors of the proposed column were smaller than that of the reference column. Therefore, the microfabricated columns described in this paper will be very useful in 10

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the development of miniaturized GC instruments. Acknowledgment

The authors greatly acknowledge the financial support from the National Science

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Foundation of China under Grant numbers: 61176112, 60976088, 31100820 and

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60701019.

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References

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chromatography column with an embedded thermal conductivity detector, IEEE

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[7] V. R. Reid, M. Stadermann, O. Bakajin, R. E. Synovec, High-speed, temperature programmable gas chromatography utilizing a microfabricated chip with an improved

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carbon nanotube stationary phase, Talanta 77(4) (2009) 1420-1425. [8] M. Stadermann, A.D. McBrady, B. Dick, V.R. Reid, A. Noy, R.E. Synovec, O.

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Bakajin, Ultrafast gas chromatography on single-wall carbon nanotube stationary

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phases in microfabricated channels, AnalChem 78(16) (2006) 5639-5644.

[9] P. R. Lewis, R.P. Manginell, G.C. Frye-Mason, Recent advances in the gas-phase

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MicroChemLab, IEEE Sens. J 6(2006) 784-795.

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[10] R. Lobinski, V. Sidelnikov, Y. Patrushev, I. Rodriguez, A. Wasik, Multicapillary column gas chromatography with element-selective detection, TrAC Trends in

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Analytical Chemistry 18 (1999) 449-460.

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gas chromatography columns, Sensors and Actuators B: Chemical 141(1) (2009) 309-315.

[12] J.H. Sun, D.F. Cui, X. Chen, L.L. Zhang, H Li, Fabrication and characterization of MEMS-based gas chromatography column with embedded micro-posts for separation of environmental carcinogens, J. Chromatogr. A 1291(24) (2013), 122. [13] O. Tobail, Z. Yan, M. Reuter, J. H. Werner, Lateral homogeneity of porous silicon for large area transfer solar cells, Thin Solid Films 516 (2008) 6959-6962 . [14] R. Herino, G. Bomchil, K. Barla, C. Bertrand, and J. L. Ginoux, Porosity and pore

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134(8) (1987) 1994-2000. [15] A. Halimaoui, Porous silicon science and technology, Springer-Verlag, 1995.

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[16] M. J. E.Golay, The height equivalent to a theoretical plate of retention less rectangular tubes, J Chromatogr. A 216 (1981) 1-8.

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[17] H.M. McNair, J.M. Miller, Basic gas chromatography, New York:

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Table Caption

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Table 1. Comparison of the peak tailing of the two columns.

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Figure Captions

Figure.1. The side view of the micro GC column based on porous silicon.

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Figure.2. The SEM micrograph of the micro channels and the micro-pillars.

Figure.3.The setup of fabrication of the porous silicon on the surface of micro

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channels and micro-pillars.

Figure.4. Interface between porous silicon and silicon substrate, (a) the porous silicon

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thickness on the micro-pillars (the porous silicon on the upper micro-pillars was etched away by 1% KOH solution), (b) the porous silicon thickness on the channels

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wall. The porous silicon with a thickness of 4 μm and a porosity of 80%.

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Figure.5. (a) the heaters and T-sensors on the backside of the GC column, and (b) photograph of the fabricated micro GC column, the length and width of the chip are

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35 mm and 35 mm, respectively.

Figure.6. Calculated height equivalent to a theoretical plate versus average carrier gas velocity for the proposed column and the reference column. Figure. 7. Separation of an environmental sample mixture of 6 compounds (benzene, toluene, ethylbenzene, styrene, nonane and decane) by the fabricated column (a) and the reference column (b).

Figure. 8. Repeatability of the column efficiency of the proposed column.

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Biographies Jianhai Sun received his Ph.D degree in electrical engineering from the Institute of

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Electronics, Chinese Academy of Sciences, Beijing, China, in 2006. His current

resonance (SPR) biochemical analysis system.

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research interests are MEMS based Gas Chromatography systems, surface plasma

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Dafu Cui graduated from the Department of Radio and Electronics, University of

Science and Technology of China in 1964. Since 1964, he has been working at

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Institute of Electronics, CAS. His current research interests are MEMS based Gas

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Chromatography systems, surface plasma resonance (SPR) biochemical analysis

technologies.

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systems and Micro Total Analytical Systems based on MEMS and NEMS

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Fengying Guan received her Ph.D degree in Beijing Foresty University in 2004.

Since 2004, she has been working at International Centre for Bamboo and Rattan. Her current research interest is forest resource monitoring.

Xing Chen received her Ph.D degree in electrical engineering from the Institute of

Electronics, Chinese Academy of Sciences, Beijing, China, in 2007. Her current research interests are Micro Total Analytical Systems based on MEMS and NEMS technologies.

Lulu Zhang received the M.S. degree in electrical engineering from the Institute of

Electronics, Chinese Academy of Sciences, Beijing, China, in 2007. Her current 16

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research interest is surface plasma resonance (SPR) biochemical analysis system.

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The calculated tailing factors ethylbenzene

styrene

nonane

decane

1.07

1.04

1.04

1.05

1.09

1.08

1.13

1.16

1.09

1.08

1.11

1.14

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toluene

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The proposed column The reference column

benzene

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Figure.1

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Figure.2

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Figure.3

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Figure.4

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Figure.5

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Figure.6

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Figure.7

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Figure.8

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