Smart Hydrogel Gratings for Sensitive, Facile and Rapid Detection of

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Materials and Interfaces

Smart Hydrogel Gratings for Sensitive, Facile and Rapid Detection of Ethanol Concentration Han-Yu Peng, Wei Wang, Fuhua Gao, Shuo Lin, XiaoJie Ju, Rui Xie, Zhuang Liu, Yousef Faraj, and Liang-Yin Chu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Industrial & Engineering Chemistry Research

Smart Hydrogel Gratings for Sensitive, Facile and Rapid Detection of Ethanol Concentration Han-Yu Peng,† Wei Wang,*,†,‡ Fu-Hua Gao,§ Shuo Lin,† Xiao-Jie Ju,†,‡ Rui Xie,†,‡ Zhuang Liu,†,‡ Yousef Faraj,†,‡ and Liang-Yin Chu*,†,‡

†School

of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P.

R. China ‡State

Key Laboratory of Polymer Materials Engineering, Sichuan University,

Chengdu, Sichuan 610065, P. R. China

§School

of Physics, Sichuan University, Chengdu, Sichuan 610065, P. R. China

1

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ABSTRACT:

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A simple grating system based on poly(N-isopropylacrylamide-co-

acrylamide) (poly(NIPAM-co-AAm)) hydrogel for sensitive and rapid detection of ethanol concentration is developed.

The hydrogel gratings enable ethanol-induced

shrinking, thus changing their refractive index and height.

The changes in the optical

property and structure of hydrogel gratings allow efficient conversion and amplification of the signal of ethanol concentration into changes of diffraction efficiency for facile detection of ethanol concentration via a simple optical detection system.

By adjusting

the molar ratio of NIPAM and AAm in the hydrogel gratings, significant diffraction efficiency changes in response to ethanol concentration in the range of 0 ~ 30 vol% at different temperatures can be achieved for sensitive ethanol detection.

Moreover, the

hydrogel gratings with nanometer-sized height and uniform surface relief structures allow rapid response time (less than 2 min) and good repeatability for ethanol detection. KEYWORDS:

hydrogel gratings; smart hydrogels; diffraction; sensitive detection;

ethanol concentration

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INTRODUCTION Ethanol plays an important role in various fields, including chemical, pharmaceutical and energy industries.1

During the biological fermentation process for ethanol

production, the efficiency and yield are limited by the tolerance capability of yeast against ethanol concentration.2

The optimum ethanol concentration in fermentation

broth is reported to be ~10 vol%,3 although this value is dependent on the process conditions.

Increase of ethanol concentration above the optimum value in

fermentation broth can lead to inactive yeast, thus causing low efficiency and low yield for ethanol production.

Therefore, the ethanol concentration during fermentation

process needs to be detected and controlled within an appropriate range for efficient production. Generally, techniques such as pycnometer method, alcoholmeter method, potassium dichromate colorimetric method,4-6 Fourier transform infrared spectrometry,7-9 and high-performance liquid chromatography,10-14 and gas chromatography,15-17 are widely used for ethanol detection.

However, the methods based on pycnometer and

alcoholmeter require time-consuming sample distillation process, and the method based on potassium dichromate oxidation colorimetry suffers from limited accuracy due to the interference of reducing substances such as glucose.

The methods based on

Fourier transform infrared spectrometry, and high-performance liquid chromatography and gas chromatography usually require expensive bulky instruments, professional operators and long detecting time.

With stimuli-responsive volume phase

transitions,18, 19 smart polymeric materials can convert various stimuli into different

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outputs such as optical,20-24 flux-based25-28 and electrical signals29, 30 showing flexible signal transduction for facile detection.

Thus, development of detection systems

based on stimuli-responsive polymeric materials creates opportunities for sensitive, facile and rapid detection of the threshold concentration of ethanol during biological fermentation process. Up to now, based on stimuli-responsive polymeric materials, detection systems such as photonic crystals,31-34 smart membranes in microchips35 and piezoresistive pressure sensors,36-39 have been developed for detecting the threshold concentration of ethanol for biological fermentation.

With ethanol-responsive volume changes, photonic

crystal hydrogels based on poly(vinyl alcohol),31 poly(N-isopropylacrylamide) (PNIPAM),32 polyacrylamide (PAAm)33 and poly(hydroxyethyl methacrylate),34 can change their lattice constants and lead to shift of diffraction peaks for ethanol detection. Such a system can sensitively detect the threshold concentration of ethanol (10 vol%), but preparation of stable photonic crystals with uniform structure and good performance usually remains challenging.

By incorporating a piece of

polyethersulfone membrane containing ethanol-responsive PNIPAM nanogels as nanovalves for flux regulation into polydimethylsiloxane microchips, rapid and sensitive detection of ethanol concentration (10 vol%) in real fermentation broth can be achieved.35

The detection results were almost the same as that detected by gas

chromatography, indicating the good performance of PNIPAM-based detectors for detecting ethanol concentrations in real fermentation broths.

However, the microchip

usually requires time-consuming fabrication, and suffers from swollen problem when


4

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contacting with ethanol.40

Piezoresistive pressure sensors integrated with PAAm36, 37

and PNIPAM38, 39 hydrogels can transform the ethanol-induced shrinking of hydrogel into resistance changes of piezoresistor for detecting the threshold concentration of ethanol (10 vol%), but they require at least 10 min for ethanol detection.

Therefore,

development of a detection platform for sensitive, facile and rapid detection of ethanol concentration during biological fermentation process still remains highly desirable. Here

we

report

on

a

diffraction

grating

system

based

on

poly(N-

isopropylacrylamide-co-acrylamide) (poly(NIPAM-co-AAm)) hydrogels for sensitive, facile and rapid detection of the threshold concentration of ethanol during biological fermentation process.

The PNIPAM and PAAm both are swollen in water due to the

hydrogen bonds formed between water molecules and amide groups in the polymers. When contacting with ethanol, the ethanol molecules can compete for water molecules with polymer chains to form stable water clathrate hydrate structures, resulting in hydrogen bonds break and hydrogel shrinking.41-45

Such a shrinking can lead to

changes of refractive index and height of hydrogel gratings to output a diffraction efficiency (DE) change for facile ethanol detection via a simple optical system.46-51 Meanwhile, based on the tunable ethanol-responsive volume changes of PNIPAM at different temperatures,52 combination of NIPAM and AAm in the hydrogel gratings enables manipulation of the change behaviors of refractive index and height of hydrogel gratings in response to changes of ethanol concentration.

Thus, significant changes

of refractive index and height of hydrogel gratings at different temperatures can be achieved to satisfy the need to sensitively detect ethanol concentration at different


5


temperatures for biological fermentation.53-58

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Moreover, the hydrogel gratings with

nanometer-sized height and uniform surface relief structures enables fast and reversible ethanol-induced volume shrinking for rapid and repeatable ethanol detection.

To

demonstrate the excellent performance for ethanol detection, hydrogel gratings with a selected composition is used for sensitive, facile and rapid detection of the threshold concentration of ethanol at 31 oC, which is the typical temperature for industrial ethanol fermentation based on mesophilic microorganisms.54

EXPERIMENTAL SECTION Materials.

N-isopropylacrylamide (NIPAM, TCI, Japan) was purified by

recrystallization with a hexane/acetone mixture (50/50, v/v).

Acrylamide (AAm,

Chengdu Kelong, China) was the monomer to copolymerize with NIPAM.

N,N’-

methylene-bis-acrylamide (MBA, Sigma-Aldrich, United States) was used as crosslinker.

2,2’-azobis (2-amidinopropane dihydrochloride) (V50, J & K Scientific Ltd,

China) was used as photo-initiator.

Anhydrous ethanol was obtained from

Commercial Alcohols (Chengdu Kelong, China). oC)

Deionized water (18.2 MΩ at 25

from a Milli-Q Plus water purification system (Millipore) was used throughout the

experiments. Preparation of poly(NIPAM-co-AAm) Hydrogel Gratings.

Ethanol-responsive

poly(NIPAM-co-AAm) hydrogel gratings with different NIPAM contents were fabricated by using microcontact printing technique.59-62

First, a grating master

(Photoresists) was prepared by interference lithography technique.63


For fabricating

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the polydimethylsiloxane (PDMS) stamp, the elastomer base and curing agent (Sylgard 184, Dow Corning) with mass ratio of 10:1 were mixed to prepare PDMS prepolymer. By casting the PDMS prepolymer against the masters and then curing in the mould at 95 oC for 1 h, a PDMS stamp was obtained.

Next, the PDMS stamp was cut into cubes

with length of 11 mm, width of 9.5 mm and height of 3 mm, for synthesis of the hydrogel grating via microcontact printing.

Typically, deionized water containing

NIPAM, AAm, MBA, and V50 was used as precursor solution for synthesizing the hydrogel grating.

Briefly, the precursor solution dropped on a glass substrate was

sandwiched between the glass substrate and PDMS stamp.

The sandwiched precursor

solution was polymerized via UV irradiation for 15 min in an ice-water bath.

After

removing the PDMS stamp and water washing, poly(NIPAM-co-AAm) hydrogel grating integrated on the glass substrate was fabricated.

The concentration of total

monomers in the reaction solution was 1 mol L−1.

MBA and V50 in the total

monomers were 2 mol% and 1.9 mol% respectively.

The feed molar ratio (RF) of

NIPAM to the total monomers was varied from 10 mol% to 100 mol%. Morphological and Structural Characterization of poly(NIPAM-co-AAm) Hydrogel Gratings.

The morphology and structure of poly(NIPAM-co-AAm)

hydrogel gratings were characterized by using digital camera, scanning electronic microscopy (SEM, G2 Pro, Phenom) and atomic force microscopy (AFM, MultiMode 8, Bruker). Setup of Ethanol-Detection Platform.

The poly(NIPAM-co-AAm) hydrogel

grating was integrated with a diode laser beam (LDM635, Thorlabs), two silicon


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photodiodes (DSi200, Zolix), a sample cell, a heating stage (TS62, Instec), and a dataanalyzing system (DCS300PA, Zolix) to construct the ethanol-detection platform.

All

the components were placed on an optical table (OTBP812-100-1, Zolix) to ensure stable detection conditions.

The hydrogel grating integrated on the glass plate was

vertically immersed in the sample cell containing aqueous solution with certain ethanol concentration.

A heating stage was used to control the temperature of the sample cell.

The diode laser beam was used to irradiate an incident light perpendicularly on the hydrogel grating; meanwhile, the two silicon photodiodes were used to detect the diffraction intensities of the zero-order (I0) and first-order (I1) diffraction beams through the hydrogel grating.

The detected signals from the two silicon photodiodes

were analyzed by the computer-coupling data-analyzing system for readout.64 Determination of the Optimal Composition of poly(NIPAM-co-AAm) Hydrogel Gratings for Sensitive Ethanol Detection. To determine the optimal composition of poly(NIPAM-co-AAm) hydrogel gratings for detecting the threshold concentration of ethanol (10 vol%) for biological fermentation, effects of composition of hydrogel gratings and operation temperature on the ethanol-concentration-dependent DE change behaviors were investigated by using deionized water and ethanol-containing solution with different ethanol concentrations (10 vol%, 20 vol%, and 30 vol%) as samples.

It

has been reported that, with increasing the ethanol concentration, the PNIPAM-based hydrogel collapses first and then reswells again.

According to previously published

works, the reentrant swelling of PNIPAM-based hydrogel occurs when the ethanol concentration exceeds an upper critical concentration around 37 vol%.45


When the

8

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ethanol concentration is below this upper critical concentration, the refractive index of hydrogel grating (ng) increases and the height of hydrogel grating (h) decreases with increasing the ethanol concentration.

However, when the ethanol concentration is

above this upper critical concentration, the ng value decreases and the h value increases with increasing the ethanol concentration.

This phenomenon will result in a DE value

corresponding to two ethanol concentrations, thus the DE value and the ethanol concentration are not a monotonic relationship.

This work aims to detect the threshold

concentration of ethanol during biological fermentation (10 vol%); therefore, we chose the ethanol concentrations ranging from 0 to 30 vol%, which were below the abovementioned upper critical concentration. changed from 13 oC to 55 oC.

The temperature of the sample cell was

At each temperature, the diffraction intensities of

hydrogel gratings with different RF in deionized water and ethanol solution were measured by the photodiodes for determining the DE changes. Test of Response Time and Repeatability for Ethanol Detection.

The response

time for ethanol detection was investigated by monitoring the dynamic DE change of hydrogel grating with RF of 100 mol% after switching deionized water to ethanolcontaining solutions with different concentrations at 31 oC.

The repeatability for

ethanol detection was investigated by using hydrogel grating with RF of 100 mol% to repeatedly detect the threshold concentration of ethanol (10 vol%) at 31 oC.

The

repeated detection was achieved by simply washing hydrogel gratings with deionized water for ethanol removal, followed with addition of ethanol-containing solutions for detection.

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RESULTS AND DISCUSSION Strategy for Designing Smart Hydrogel Gratings for Ethanol Detection.

Figure

1 shows the design strategy of the hydrogel gratings with poly(NIPAM-co-AAm) networks (Figure 1a) for sensitive, facile and rapid detection of ethanol concentration. In this poly(NIPAM-co-AAm) hydrogel gratings, both NIPAM and AAm act as ethanol-responsive sensors.

When placed in water, the hydrogel gratings are swollen

due to the hydrogen bonds formed between water molecules and amide groups in their poly(NIPAM-co-AAm) networks.

In the presence of ethanol, because ethanol

molecules can compete with poly(NIPAM-co-AAm) for water molecules to form stable water clathrate hydrates, the hydrogel gratings shrink due to the breakage of hydrogen bonds and increase of free energy for polymer-polymer contact.41-45

Due to the

ethanol-responsive volume changes, the refractive index and height of the hydrogel grating can be adjusted.

This allows change of the intensities of the first-order (I1)

and zero-order (I0) diffraction beams through the hydrogel grating, leading to variation of the first-order diffraction efficiency (DE=I1/I0) (Figure 1b).46-51

The values of I1

and I0 of hydrogel grating fixed in a transparent cell can be measured by using an optical detection platform equipped with a He-Ne laser, a photodiode and a data acquisition system for detecting and analyzing the optical signals (Figure 1c).

Thus, based on the

optical detection platform, the changes of DE value can be obtained from the measured values of I1 and I0.

Moreover, since the characteristic shrinking time is proportional

to the square of hydrogel dimension,65, 66 the hydrogel grating with nanometer-sized

10


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height and uniform surface relief structures allows rapid and reversible volume shrinking in response to ethanol concentration changes.

Therefore, the proposed

system based on poly(NIPAM-co-AAm) hydrogel grating enables sensitive, facile and rapid detection of ethanol concentration. Fabrication of poly(NIPAM-co-AAm) Hydrogel Gratings via Microcontact Printing.

The poly(NIPAM-co-AAm) hydrogel gratings are prepared by using

microcontact printing technique,59-62 as schematically illustrated in Figure 2.

A

PDMS stamp is used to shape the precursor solution for preparing the hydrogel gratings. The precursor solution contains monomers NIPAM and AAm, cross-linker MBA, and photo-initiator V50 to construct the crosslinked poly(NIPAM-co-AAm) networks. Briefly, the precursor solution is dropped onto a glass substrate (Figure 2a), and then covered with the PDMS stamp, followed with UV-initiated polymerization in an icewater bath (Figure 2b).

After carefully peeling off the stamp, the poly(NIPAM-co-

AAm) hydrogel grating fixed on the glass substrate is obtained (Figure 2c).

The

hydrogel grating is immersed in deionized water for 24 h to remove the unreacted monomers, cross-linker and photo-initiator before further use. Figure 3a shows the optical image of poly(NIPAM-co-AAm) hydrogel grating, fabricated by using precursor solution with RF of 90 mol%.

A clear diffraction

phenomenon can be observed on the hydrogel grating with RF of 90 mol%, due to its uniform and regular surface relief structures (Figure 3a).

The SEM and AFM images

also confirm the surface relief structures of the hydrogel grating (Figure 3b-c), which is important for the hydrogel grating to achieve sensitive and repeatable detection


11


performances.

Page 12 of 40

To investigate the height of the hydrogel grating, the section analysis

of hydrogel grating in dried and hydrated states are tested by AFM at 25 oC.

The

height of the hydrogel grating in dried state is ~7 nm (Figure 3c), while that of the hydrogel grating in hydrated state is approximately ~200 nm (Figure 4a), due to the uptake of a large amount of water in its three-dimensional polymeric networks.67 However, because the hydrogel grating is fixed on the glass substrate, the widths of the hydrogel grating in dried (Figure 3c) and hydrated (Figure 4a) states at 25 oC remains unchanged (~1220 nm).

Moreover, the changes of height and width of the hydrogel

gratings in aqueous solutions with different ethanol concentrations at 25 oC are investigated (Figures 4 and 5).

With increasing the ethanol concentration from 0 vol%

to 30 vol%, the height of hydrogel gratings decreases from 201 nm to 97 nm (Figures 4 and 5a).

Meanwhile, the width of the hydrogel gratings remains almost the same

due to the fixation of hydrogel grating on the glass substrate (Figures 4 and 5b).

The

results confirm the only change of height of hydrogel gratings during their ethanolresponsive volume phase transitions. Temperature- and Ethanol-Responsive Characteristics of poly(NIPAM-coAAm) Hydrogel Gratings.

Since the poly(NIPAM-co-AAm) hydrogel grating can

show volume changes when responding to changes of temperature and ethanol concentration, the effects of temperature and ethanol concentration on the DE values are investigated by using the optical detection platform.

As shown in Figure 6, for

hydrogel gratings with RF ≥ 70 mol%, the DE values in deionized water increase with increasing temperature from 13 oC to 55 oC.

Moreover, the volume phase transition


12

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temperature (VPTT) of poly(NIPAM-co-AAm) hydrogel grating, defined as the temperature at which the hydrogel grating shows the most dramatic optical signal changes, decreases with increasing RF, and the maximum value of DE increases.

On

the contrary, for hydrogel gratings with RF < 70 mol%, the DE values in deionized water decrease with increasing temperature from 13 oC to 55 oC. According to the grating theory, the DE value is proportional to the square of the refractive index contrast between the hydrogel grating and surrounding medium, and the square of the height of hydrogel grating.68-70

The relationship among these factors

as mentioned above can be approximated by the following equation:

   ng  ns  h  DE    2  

2

(1)

where, ng and ns are respectively the refractive indexes of poly(NIPAM-co-AAm) hydrogel grating and the surrounding solution; h is the height of poly(NIPAM-co-AAm) hydrogel grating in the surrounding solution.

With increasing temperature, the

hydrogel grating shrinks and results in increase of ng and decrease of h for PNIPAMbased hydrogel networks.71, 72

Meanwhile, the ns value of deionized water decreases

slightly with increasing temperature.73

According to Equation (1), the increase of ng

and decrease of ns enable a positive effect on enhancement of DE changes, while the decrease of h enables a negative effect.

By tuning the molar ratio of NIPAM and

AAm in the hydrogel grating, the changes of ng and h can be adjusted to manipulate the competition between their positive and negative effects to achieve an enhanced DE change.

As shown in Figure 6, for hydrogel gratings with RF 70 mol%, their DE values in deionized water largely increase with increasing temperature, indicating that the positive effects of increased ng and decreased ns on increasing DE values dominate.

Moreover, with increasing RF,

the contents of NIPAM in the hydrogel gratings increase, leading to more dramatic volume transitions as well as more significant DE changes around their VPTT.

Since

the VPTT can be adjusted by changing RF, the temperature at which the DE values can show significant changes can be flexibly tuned to meet the demands for sensitive detection of threshold concentration of ethanol at different temperatures. Because the hydrogel gratings with RF >70 mol% can show significant DE changes upon volume changes, these hydrogel gratings are further used for studying their DE changes in response to changes of ethanol concentration.

Figures 7a-d show the DE

values of poly(NIPAM-co-AAm) hydrogel gratings with RF of 75 mol%, 80 mol%, 90 mol%, and 100 mol% in ethanol solutions with concentrations varying from 0 vol% to 30 vol% at different temperatures.

For the hydrogel gratings with RF of 75 mol%

(Figure 7a) and 80 mol% (Figure 7b), at each temperature, only slight changes are observed for their DE values at ethanol concentrations of 0 vol% and 10 vol%, indicating poor sensitivity for detection of 10 vol% ethanol. For the hydrogel gratings with RF of 90 mol% (Figure 7c) and 100 mol% (Figure 7d), upon change of ethanol concentrations from 0 vol% to 10 vol%, significant changes of DE values can be obtained at 40 oC for hydrogel grating with RF of 90 mol% and at 31 oC for hydrogel

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The temperature of 31 oC is the typical temperature for

grating with RF of 100 mol%.

industrial ethanol fermentation using mesophilic microorganisms,54 while the temperature of 40 oC is usually required by high-temperature fermentation (≥ 40 oC) that allows improved ethanol fermentation, reduced cooling costs and contamination prevention.53

This provides the opportunity for the hydrogel gratings to achieve

sensitive detection of 10 vol% ethanol at tunable operation temperatures.

Moreover,

based on Figure 7, the corresponding contour diagram for ethanol concentration is shown in Figure 8.

Therefore, according to Figure 8, ethanol concentration can be

determined with measured values of the solution temperature and DE of poly(NIPAMco-AAm) hydrogel gratings. Sensitive, Rapid and Repeatable Detection of the Threshold Concentration of Ethanol during Biological Fermentation.

To achieve sensitive detection of the

threshold concentration of ethanol (10 vol%) at 31 oC, poly(NIPAM-co-AAm) hydrogel grating with RF of 100 mol% is used as the detection system.

As shown in

Figure 9, at 31 oC, the DE values of the hydrogel grating in aqueous solutions with 4 vol% and 7 vol% ethanol are nearly the same (~1.016) as that in deionized water (0 vol% ethanol).

When increasing ethanol concentration to 10 vol%, the DE value shows a

significant increase to 1.539.

Such a detection performance benefits the use of the

hydrogel grating systems for sensitive detection of threshold concentration of ethanol during biological fermentation.

Moreover, since change of RF in hydrogel gratings

allows flexibly adjusting the temperature at which significant DE changes can be 15



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obtained in response to 10 vol% ethanol (Figure 7c-d), detection of threshold concentration of ethanol at different temperatures can be achieved. Figure 10 shows the time-dependent dynamic changes of DE values of poly(NIPAMco-AAm) hydrogel grating with RF of 100 mol% in aqueous solutions with different ethanol concentrations.

Upon addition of the ethanol-containing solutions into the

cells containing the hydrogel grating, the DE values show a fast increase at the beginning.

As time increases, the DE value for all hydrogel gratings reach an

equilibrium value after ~90 s, indicating the fast volume shrinking of hydrogel gratings in response to changes of ethanol concentrations.

Such a fast response is due to the

nanometer-sized height of hydrogel gratings, because the shrinking time is proportional to the square of the characteristic dimension of hydrogel.65, 66

Thus, based on the fast

response of hydrogel gratings in response to ethanol concentration changes, the ethanol concentration during biological fermentation process can be detected rapidly by our proposed hydrogel grating system.

Moreover, the reversible volume phase transitions

of PNIPAM-based hydrogel upon changes in temperature and ethanol concentration, enable repeated use of the hydrogel gratings for ethanol detection.

As shown in Figure

11, with repeated change of the ethanol concentration between 0 vol% and 10 vol % for several cycles, the hydrogel grating exhibits repeatable DE values for each of the ethanol concentration.

All the results indicate the good performance of the hydrogel

gratings for sensitive, rapid and repeatable ethanol detection.

CONCLUSIONS

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In summary, simple grating systems based on poly(NIPAM-co-AAm) hydrogel for sensitive and rapid detection of ethanol concentration has been developed.

The

hydrogel gratings with nanometer-sized height are fabricated by a simple one-step process based on microcontact printing technique.

By integrating the hydrogel

grating into a simple optical detection system, the signal of ethanol concentration can be efficiently converted and amplified into optical intensity signal.

The hydrogel

gratings with different compositions of NIPAM and AAm can achieve significant DE changes when responding to ethanol concentration in the range of 0 ~ 30 vol% at different temperatures, for sensitive and rapid ethanol detection within 2 min. Therefore, the proposed hydrogel grating system provides an efficient strategy to achieve sensitive, facile and rapid detection of ethanol concentration, which is highly promising for practical applications in ethanol separation and fermentation.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors.

All authors have

given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. 17



Page 18 of 40

ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202, 21576167, 81621062), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

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Figures

Figure 1. Schematic illustration of ethanol-responsive hydrogel grating system for sensitive, facile and rapid ethanol detection. co-AAm) hydrogel grating.

(a) Molecular structure of poly(NIPAM-

(b) Changes of height and refractive index of

poly(NIPAM-co-AAm) hydrogel grating during its ethanol-concentration-induced volume shrinking process.

(c) Ethanol-detection system containing the poly(NIPAM-

co-AAm) hydrogel grating, a He-Ne laser, a photodiode and a data acquisition system integrated with computer.

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Figure 2. Schematic illustration of fabrication process of poly(NIPAM-co-AAm) hydrogel grating.

(a) Drop of precursor solution onto a glass substrate.

(b) Cover of

the dropped precursor solution with a PDMS stamp for UV-initiated polymerization. (c) Removal of the PDMS stamp for fabricating the hydrogel grating integrated on the glass substrate.

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Figure 3. Optical (a), SEM (b) and AFM (c) images of poly(NIPAM-co-AAm) hydrogel gratings with RF of 90 mol% in dried state.

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Figure 4. AFM images and sectional analyses of poly(NIPAM-co-AAm) hydrogel gratings with RF of 90 mol% in aqueous solutions with ethanol of 0 vol% (a), 10 vol% (b), 20 vol% (c), and 30 vol% (d) at 25 oC.

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Figure 5. Ethanol-concentration-dependent height (h) (a) and width (w) (b) changes of poly(NIPAM-co-AAm) hydrogel grating with RF of 90 mol% at 25 oC.

The error bars

represent deviation in heights (a) and widths (b) among different grating lines.

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Figure 6. Effect of temperature on the DE change of poly(NIPAM-co-AAm) hydrogel gratings with different RF values.

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Figure 7. Temperature- and ethanol-responsive volume changes of poly(NIPAM-coAAm) hydrogel gratings with RF of 75 mol% (a), 80 mol% (b), 90 mol% (c), and 100 mol% (d).

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Figure 8.

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Contour diagrams of DE values of poly(NIPAM-co-AAm) hydrogel

gratings with RF of 75 mol% (a), 80 mol% (b), 90 mol% (c), and 100 mol% (d) as functions of both ethanol concentration and temperature.

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Figure 9. Temperature-dependent DE changes of poly(NIPAM-co-AAm) hydrogel grating with RF of 100 mol% in aqueous solutions containing ethanol with different concentrations.

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Figure 10. Time-dependent DE changes of poly(NIPAM-co-AAm) hydrogel grating with RF of 100 mol% during detection of ethanol with varied concentrations in aqueous solutions at 31 oC.

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Figure 11. Poly(NIPAM-co-AAm) hydrogel grating with RF of 100 mol% for repeated ethanol detection.

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Graphic for TOC

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Smart Hydrogel Gratings for Sensitive, Facile and Rapid Detection of

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