Visual detection of methanol in alcoholic beverages using alcohol-responsive poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) copolymers as indicators

Abstract

A simple and visual method for quantitative detection of methanol in alcoholic beverages by using alcohol-responsive poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (poly(NIPAM-co-DMAA)) linear copolymers as indicators is developed in this paper. The different number of carbon atoms in the alcohol molecules leads to the differential alcohol-responsive characteristics of the poly(NIPAM-co-DMAA) linear copolymer. With replacement of ethanol with an equal volume of methanol in alcoholic solutions, the poly(NIPAM-co-DMAA) copolymer chains change from the shrinking state to the stretching state isothermally. Therefore, the methanol concentration can be simply detected by observing the optical transmittance change of alcoholic beverages with poly(NIPAM-co-DMAA) linear copolymer as an indicator. The minimum methanol concentration that can be visually detected by using the poly(NIPAM-co-DMAA) linear copolymer containing 12.4 mol% of N,N-dimethylacrylamide is as low as 2.5 vol%. The presented detection method with the poly(NIPAM-co-DMAA) linear copolymer as indicator is quite simple and low-cost, and it is valuable for further design of simple and portable tools for home testing the general population, especially in developing countries.

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Introduction

Both ethanol and methanol are widely used in the fields of chemical, medical, and energy industries, etc. As an important component in alcoholic beverages and liquid cosmetics, ethanol is not harmful to the human body in small amounts. However, methanol will convert into strongly toxic metabolites in vivo, which damage the central nervous system and vision, and even lead to death.^1–3 The minimum toxicity dose of methanol in the adults is only several milliliters. Since ethanol and methanol have the similar physical properties, it is difficult to distinguish two of them. It has been reported that, in some undeveloped areas of Asia, Africa and Eastern Europe, methanol is sometimes illegally blended into alcoholic beverages instead of ethanol to produce low-cost products, which leads to methanol poisoning and even death. Therefore, detection of methanol concentration in alcohol beverages is significant for human health. Up to now, the methanol concentration in the alcoholic solution is generally detected by instrument analysis methods such as gas chromatography,^4–6 liquid chromatography⁷ and spectrometry.^8,9 Although these methods are precise and reliable, they are nearly impossible to be popularised in daily life, because expensive equipment as well as sophisticated and professional operations are required. Therefore, seeking for simple, convenient, low-cost analytical methods for the general population to detect methanol in alcoholic solution is particularly important.

Recently, a series of alcohol-responsive smart materials composed of poly(N-isopropylacrylamide) (PNIPAM) have been reported, such as linear polymers,^10–13 linear-grafted gating membranes,^14,15 hydrogels,^16,17 microspheres¹⁸ and microcapsules.¹⁹ All these polymeric materials reveal a reentrant swelling–shrinking–swelling phase transition with the increase of alcohol concentration. Besides, differential alcohol-responsive characteristics of the PNIPAM polymers in response to alcohol molecules with different carbon atoms numbers are observed. Swelling behaviors of PNIPAM hydrogels in methanol, ethanol, 1-propanol and 2-methyl-2-propanol aqueous solutions were studied by Mukae et al.,¹⁶ and the PNIPAM hydrogels showed analogous reentrant transition behavior in all the investigated water/alcohol mixtures. However, the mole fraction of alcohols in the alcohol/aqueous solutions at which PNIPAM hydrogels exhibited the maximum deswelling decreased as increasing the number of carbon atoms in alcohol. For instance, PNIPAM hydrogels reached to the maximum deswelling in 10 mol% of water/methanol solution while 7 mol% of water/ethanol solution. Similar results have been obtained in the microgels¹⁸ and such a differential alcohol-responsive characteristic is attributable to the different abilities of alcohol molecules to form hydrated clathrate structure with water molecules. Zhu et al.²⁰ investigated the coil-to-globule transition of PNIPAM chains in methanol, ethanol and 2-propanol aqueous solutions with alcohol concentration range from 0 to 100 vol%. Similarly, with increasing the alcohol volume fraction, a sharper decrease in the chain length of PNIPAM was found as the number of carbon atoms in the alcohols increased from 1 to 3. Recently, Liu et al.¹⁹ has developed an alcohol-responsive PNIPAM microcapsule to find the lower critical solution temperature (LCST) changing with increasing the volume fraction of methanol and ethanol in the aqueous solution. The LCST values of PNIPAM microcapsules decreased faster with an increase of the ethanol fraction rather than the methanol fraction. Therefore, at the same volume fraction, the PNIPAM microcapsules revealed a lower LCST values in water/ethanol solution than that in water/methanol solutions. Alcohol-responsive polymeric materials composed of PNIPAM revealed a higher sensitivity to alcohol containing more carbon atoms, which was expected to quantitatively reflect the concentration of ethanol and methanol in alcoholic solutions. The differential alcohol-responsive characteristics provide the methodological and theoretic fundaments to distinguish methanol from ethanol. However, previous investigations are mainly focused on binary solution systems of single alcohol with water. Up to now, in-depth research and systematic study in water/methanol/ethanol solutions is still lacked for the simple and low-cost methanol detection by using PNIPAM-based polymers as indicators, although such investigations could provide valuable fundamental principles for further designing simple and portable tools for daily methanol detection.

Herein, we report on the visual detection of methanol in alcoholic beverages by using alcohol-responsive poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (poly(NIPAM-co-DMAA)) copolymers as indicators. The differential alcohol-responsive characteristics of (poly(NIPAM-co-DMAA)) linear copolymers with different molar fractions of N,N-dimethylacrylamide (DMAA) in response to ethanol and methanol are investigated systematically. PNIPAM-based polymeric chains are stretched in aqueous solution due to the hydrogen bonds formed between water molecules and amide groups in the polymers at temperatures lower than the LCST. However, in alcohol aqueous solution at the same temperature, the alcohol molecules compete for water molecules with PNIPAM chains to form water clathrate hydrate structures. Thus the PNIPAM-based polymeric chains shrink in alcohol aqueous solution at the same temperature due to the broken hydrogen bonds.^11,12,21 The maximum number of water molecules that per mole of alcohol could bind varies with the number of carbon atoms, which is 12 for methanol while 17 for ethanol.²² Therefore, compared with methanol molecules, ethanol molecules have stronger competitiveness for water molecules with PNIPAM-based polymeric chains, and can trigger the coil-to-globule transition of PNIPAM-based polymer more easily. Thus, the PNIPAM-based polymer is more sensitive to ethanol rather than methanol.^16,18–20 Hydrophilic monomer DMAA is copolymerized with PNIPAM to adjust the LCST value (LCST_i) to approach the appropriate operation temperature (T₀).^23–25 As shown in Fig. 1a, when ethanol in aqueous solution is replaced by equal volume of methanol, the LCST of the poly(NIPAM-co-DMAA) linear copolymer shifts from LCST₁ to LCST₂ (LCST₁ < LCST₂). The LCST_i of the poly(NIPAM-co-DMAA) linear copolymer in water/methanol/ethanol ternary solution changes reversibly between LCST₁ and LCST₂ depending on how much ethanol is replaced by methanol. When the operation temperature (T₀) is located between LCST₁ and LCST₂, the poly(NIPAM-co-DMAA) copolymer chains exhibit an isothermal and reversible stretching/shrinking transition with the variation of methanol concentration due to differential alcohol-responsive characteristics in response to ethanol and methanol. The poly(NIPAM-co-DMAA) copolymer chains shrink in the solution with no or little methanol, and thus the ternary alcohol solution looks cloudy. However, the poly(NIPAM-co-DMAA) copolymer chains stretch in the solutions with more methanol, and the alcohol solution becomes transparent (Fig. 1b). Therefore, the transparency of the water/methanol/ethanol ternary solution containing poly(NIPAM-co-DMAA) linear copolymers could directly tell the concentration of methanol in the alcohol solutions.

Fig. 1 Schematic illustration of methanol detection with poly(NIPAM-co-DMAA) copolymers. (a) The LCST_i of PNIPAM-based polymers shifts from LCST₁ to LCST₂ when ethanol in solutions is replaced by equal volume of methanol, in which C_E and C_M are concentrations of ethanol and methanol respectively. (b) Methanol-concentration-dependent and temperature-dependent solubility change of the poly(NIPAM-co-DMAA) copolymers in alcohol solution caused by the configuration change, as well as the isothermal appearance change of the solution at room temperature T₀ (on the dotted line).

Experimental

Materials

N-Isopropylacrylamide (NIPAM, Sigma-Aldrich) is purified by recrystallization with a hexane/acetone mixture (50/50, v/v). N,N-Dimethylacrylamide (DMAA, Sigma-Aldrich) is the monomer to copolymerize with NIPAM and generate poly(NIPAM-co-DMAA) linear copolymers. 2,2′-Azobisi-sobutyronitrile (AIBN) is recrystallized with ethanol and used as the initiator. All other chemicals are of analytical grade and used as received. Deionized water (18.2 MΩ, 25 °C) from a Milli-Q Plus water purification system (Millipore) is used throughout the experiments.

Preparation and componential characterization of poly(NIPAM-co-DMAA) linear copolymers

A series of poly(NIPAM-co-DMAA) linear copolymers with varying DMAA contents are synthesized by thermally initiated free-radical copolymerization in tetrahydrofuran (THF). The concentration of NIPAM in THF is fixed at 0.3 mol L⁻¹, and the feed molar ratios of DMAA to the total monomers are designed as 6.3, 7.4, 9.1, and 10.3 mol%, respectively. The molar ratio of the initiator AIBN to NIPAM is 1 : 150. The copolymerization is carried out under magnetic stirring and nitrogen atmosphere in a thermostatic water bath at 65 °C for 12 h. The resultant poly(NIPAM-co-DMAA) linear copolymers are purified three times by reprecipitation with an excess of anhydrous ethyl ether to thoroughly remove the unreacted monomers. The purified copolymers are dried under vacuum at 50 °C for 12 h.

The chemical composition of purified poly(NIPAM-co-DMAA) linear copolymers is characterized by nuclear magnetic resonance spectrometry (¹H NMR, Varian-400, Varian). The molecular weights of poly(NIPAM-co-DMAA) linear copolymers with different DMAA contents are determined by gel permeation chromatography (GPC, Waters-2410, Waters) using THF as the mobile phase and polystyrene as the standard.

Investigation of differential alcohol-responsive characteristics of poly(NIPAM-co-DMAA) linear copolymers in response to ethanol and methanol

The alcohol-responsive behaviors of poly(NIPAM-co-DMAA) linear copolymers are investigated by measuring the LCST values in water/ethanol binary solutions, water/methanol binary solutions, and water/methanol/ethanol ternary solutions with different concentrations. The concentrations of alcohols are in a range from 0 to 40 vol%. The optical transmittances of the alcoholic solutions of copolymers are measured as a function of temperature using a UV-vis spectrophotometer (UV-1700, Shimadzu) at a wavelength of 500 nm, which is equipped with a temperature-controlled cell (TCC-240A, Shimadzu). The LCST value is defined as the temperature at which the optical transmittance of alcoholic solution containing copolymers decreases to half of the initial value.

Investigation of effects of hydrophilic monomer DMAA on the LCST values of poly(NIPAM-co-DMAA) linear copolymers in water/methanol/ethanol ternary solutions

As the alcohol concentration increasing, the LCST values of PNIPAM-based polymers in alcohol especially in ethanol shift to a lower temperature. In order to obtain the feasible operation temperature, the LCST values of PNIPAM-based polymers are adjusted by copolymerizing with hydrophilic monomer DMAA.^23–25 The LCST values of linear copolymers with different DMAA contents are evaluated in the water/methanol/ethanol ternary solutions with the total alcohol concentration of 30 vol%.

Detection of methanol concentration in simulated alcoholic beverage by using poly(NIPAM-co-DMAA) linear copolymers as indicators

Due to the differential alcohol-responsive property in response to ethanol and methanol, the poly(NIPAM-co-DMAA) linear copolymers might be applied to detect methanol concentration in water/methanol/ethanol ternary solution by simply observing the optical transmittance variation. Here, the total concentration of methanol and ethanol in the simulated alcoholic beverage is chosen as 38 vol%. The methanol concentration in the water/methanol/ethanol ternary solution varies from 0 to 38 vol%. The quantitative measurement of optical transmittance of the alcoholic beverage containing poly(NIPAM-co-DMAA) linear copolymers as a function of temperature is the same as described above. The macroscopic transmittance change of the alcoholic beverage containing poly(NIPAM-co-DMAA) linear copolymers at different operation temperatures is recorded by a digital camera. The temperatures, which are between the lower LCST in alcoholic beverage with relatively low methanol concentration and the higher LCST in alcoholic beverage with relatively high methanol concentration, are set as the operation temperatures to obtain the quantitative transmittance and to take digital photos of the alcoholic beverages. The alcoholic beverage solutions containing poly(NIPAM-co-DMAA) linear copolymers have been equilibrated adequately for 30 min at each designed operation temperature in advance to obtain a constant transmittance.

Results and discussion

Chemical composition and molecular weights of poly(NIPAM-co-DMAA) linear copolymers

The actual molar fractions of DMAA in the poly(NIPAM-co-DMAA) linear copolymers determined from the ¹H NMR results are 6.3, 7.7, 9.5 and 12.4 mol% (please see Fig. S1 in the ESI†), which are close to their feed molar fractions. According to the actual molar fractions, the series of resultant poly(NIPAM-co-DMAA) linear copolymers are labelled as PND-6.3, PND-7.7, PND-9.5 and PND-12.4, respectively. Moreover, the weight average molecular weights (M̄_w) of poly(NIPAM-co-DMAA) linear copolymers with different DMAA contents determined by GPC are in the range from 4500 to 5500 g mol⁻¹. The details of GPC results are shown in Table S1 in the ESI.† After investigating the effects of the concentrations of linear copolymers on the optical transmittance change of ethanol solutions, the concentrations of linear copolymers in the alcoholic solutions are all selected as 0.3 wt% in the subsequent experiments (please see Fig. S2 and relevant discussions in the ESI† for details).

Differential effects of ethanol and methanol on the alcohol-responsive characteristics of poly(NIPAM-co-DMAA) linear copolymers

In the water/ethanol or water/methanol binary solutions with the alcohol concentration ranging from 0 to 30 vol%, the poly(NIPAM-co-DMAA) linear copolymers (e.g., PND-6.3) exhibit gradual variation of LCST values. The LCST of PND-6.3 decreases from 36.5 °C in water to 16.1 °C in 30 vol% of ethanol aqueous solution and 29.6 °C in 30 vol% of methanol aqueous solution (Fig. 2). Obviously, the LCST values of the PND-6.3 linear copolymers always shift to a lower temperature as the alcohol concentrations increase no matter whether in ethanol or methanol aqueous solutions. Compared with those in methanol aqueous solutions, the LCST values of PND-6.3 linear copolymers reveal a much sharper decrease with an increasing alcohol concentration in ethanol aqueous solutions. Therefore, the LCST shift of the poly(NIPAM-co-DMAA) linear copolymers in ethanol aqueous solution is larger than that in methanol aqueous solution with the same alcohol concentration due to the higher sensitivity to ethanol, which is consist with the results in previous studies.^19,20 It is attributed to the differential capability of the alcohol molecules to bind water molecules. PNIPAM-based polymer chains stretch in aqueous solution with low alcohol concentration, since the hydrogen bonds between water molecules and amide groups in the polymer are formed. When alcohol concentration increases, the added alcohol molecules compete for water molecules with PNIPAM chains to form water clathrate hydrate structures. Thus, the hydrogen bonds between water molecules and polymer chains are destroyed and the polymer chains shrink. The maximum number of water molecules, which an ethanol molecule required to form the water clathrate hydrate structure, is larger than that of methanol.^20,22 Therefore, the ethanol molecule has a stronger competitiveness for water molecules with PNIPAM-based copolymer chains than the methanol molecule, and thus the PNIPAM-based polymer is more sensitive to ethanol than methanol.

Fig. 2 Differential effects of alcohol concentration on LCST of PND-6.3 linear copolymers in water/ethanol and water/methanol binary solutions. ΔLCST presents the absolute temperature difference between LCST values of poly(NIPAM-co-DMAA) linear copolymers in ethanol aqueous solution and methanol aqueous solution at the same alcohol concentration, which increases as alcohol concentration increasing.

ΔLCST is defined as the absolute temperature difference between LCST values of poly(NIPAM-co-DMAA) copolymers in ethanol aqueous solution and methanol aqueous solution at the same alcohol concentration. As shown in Fig. 2, the values of ΔLCST are respectively 1.5, 6.5 and 13.5 °C in the alcohol solutions of 10, 20 and 30 vol%, which increase as the alcohol concentration increasing and climb faster in higher alcohol concentration. Moreover, the differential alcohol-responsive characteristics of the poly(NIPAM-co-DMAA) linear copolymers in response to ethanol and methanol are observed obviously in the temperature range of ΔLCST. That is, if the operation temperature is located in the temperature range of ΔLCST, the methanol aqueous solution is easily distinguished from ethanol aqueous solution by using poly(NIPAM-co-DMAA) linear copolymers as indicators. The larger the ΔLCST is, the broader the operation temperature range, and the easier the visual observation of methanol in alcoholic solution is. The results in Fig. 2 exhibit the possibility of methanol concentration detection in water/methanol/ethanol ternary solutions.

The LCST values of PND-6.3 linear copolymers in water/methanol/ethanol ternary solutions with different methanol concentrations are shown in Fig. 3. In the alcohol solutions with the total concentrations ranging from 10 to 40 vol%, the LCST values of PND-6.3 linear copolymers increase linearly with an increase in the methanol concentration (C_M) and a decrease in the ethanol concentration (C_E) at a certain total alcohol concentration. The slopes of the LCST ∼ C_M lines of PND-6.3 linear copolymers become steeper as the total alcohol concentrations increase. The LCST value of PND-6.3 linear copolymers increases averagely by 1.6, 3.3, 4.6 and 5.9 °C when 10 vol% ethanol is replaced by methanol in alcohol solution with the total alcohol concentration of 10, 20, 30 and 40 vol% respectively. Since the larger difference in LCST values provides a wider choice of operation temperature range, the water/methanol/ethanol ternary solutions with the alcohol concentration of 30–40 vol% are more likely to obtain feasible investigation of methanol in alcoholic solution. Moreover, 30–40 vol% of alcohol concentration is also the alcohol content of most commercial alcoholic beverages.

Fig. 3 Effects of ethanol and methanol concentration on the LCST values of PND-6.3 copolymers in water/methanol/ethanol ternary solutions. The total alcohol concentration is 10 vol% (a), 20 vol% (b), 30 vol% (c), and 40 vol% (d), respectively. The LCST values increase averagely by 1.6, 3.3, 4.6 and 5.9 °C per 10 vol% ethanol replaced by methanol (on the dotted line).

Effects of hydrophilic monomer DMAA on the LCST values of poly(NIPAM-co-DMAA) linear copolymers in water/methanol/ethanol ternary solutions

The effects of hydrophilic monomer DMAA on the LCST values of poly(NIPAM-co-DMAA) linear copolymers in 30 vol% of alcohol solutions are shown in Fig. 4. Like that of PND-6.3, the LCST values of PND-7.7, PND-9.5 and PND-12.4 linear copolymers linearly increase with decreasing ethanol concentration and increasing methanol concentration. Although the slopes of the plot of LCST as the function of ethanol/methanol concentrations are almost the same, the absolute LCST values of poly(NIPAM-co-DMAA) linear copolymers with various DMAA contents are different at the same ethanol/methanol concentration. Since the M̄_w of all the poly(NIPAM-co-DMAA) linear copolymers are of an order of magnitude, the effect of molecular weights on the LCST values of such copolymers in the alcohol solution can be neglected. As the DMAA contents increase, the LCST values of poly(NIPAM-co-DMAA) linear copolymers go up correspondingly as shown in Fig. 4a. The poly(NIPAM-co-DMAA) linear copolymers with higher DMAA contents have more hydrophilic donors to form hydrogen bonds, and thus it is more difficult to destroy the interactions between the copolymers and water molecules. Therefore, higher temperature is needed to achieve the configuration transition of copolymers, and thus higher LCST values are obtained. The LCST values of PND-7.7, PND-9.5 and PND-12.4 are averagely 1.3, 5.1 and 7.2 °C higher than that of PND-6.3 over alcohol concentration of 10–30 vol%. That is, the LCST values of poly(NIPAM-co-DMAA) linear copolymers shift positively more than 1 °C when 1 mol% DMAA is added in the copolymers. Clearly, the LCST ranges of the poly(NIPAM-co-DMAA) linear copolymers can be controlled by adjusting the molar fraction of the hydrophilic monomer DMAA in the copolymer. Therefore, an isothermal configuration transition of poly(NIPAM-co-DMAA) linear copolymers as indicators in response to methanol can be easily achieved at or around room temperature, which makes it possible for the methanol detection to be operated simply under general daily conditions.

Fig. 4 Comprehensive effects of both DMAA contents and methanol concentration in 30 vol% alcohol solutions on the LCST values of poly(NIPAM-co-DMAA) copolymers. (a) Linear relationship; (b) three-dimensional diagram.

A comprehensive effect of both the DMAA contents in copolymers and the methanol concentration in 30 vol% of alcohol solutions on the LCST values of poly(NIPAM-co-DMAA) copolymers is illustrated in a 3D diagram (Fig. 4b). The LCST values of poly(NIPAM-co-DMAA) linear copolymers simply increase with increasing the DMAA contents and methanol concentration. The addition of methanol and reduction of ethanol result in the higher LCST values of poly(NIPAM-co-DMAA) linear copolymers. Meanwhile, the increase of DMAA contents in poly(NIPAM-co-DMAA) copolymers also results in an increase in the LCST values due to more hydrophilic donors to form hydrogen bonds as described above. This diagram provides valuable guidance for choosing copolymers with appropriate DMAA contents to carry out the methanol detection at a certain temperature and a certain range of methanol concentration.

Detection of methanol in simulated alcoholic beverage by using poly(NIPAM-co-DMAA) linear copolymers as indicators

The optical transmittance variations of simulated commercial alcoholic beverages as the function of methanol concentration are systematically studied by using the poly(NIPAM-co-DMAA) linear copolymers as indicators. The total alcohol concentration of the simulated alcoholic beverage is fixed at 38 vol%, while the methanol concentration varies from 0 to 38 vol%.

The temperature-dependent transmittance change of simulated alcoholic beverage containing PND-9.5 linear copolymer as an indicator is shown in Fig. 5a. The simulated alcoholic beverage varies from transparent to cloudy at temperature around the LCST of PND-9.5 no matter how large the methanol concentration is. When the temperature is lower than the LCST, the poly(NIPAM-co-DMAA) chains in the alcoholic beverage are surrounded with water molecules via hydrogen bonds and exhibit a stretching state. Thus, the poly(NIPAM-co-DMAA) linear copolymer is soluble, leading to the transparent alcohol solution. However, when the temperature is higher than the LCST, the hydrogen bonds between the copolymer chains and water molecules break, and thus the copolymer chains collapse and reveal a shrinking state instead. The alcohol solution becomes cloudy in this situation. Fig. 5b shows the LCST change of PND-9.5 linear copolymer in the simulated alcoholic beverage as the function of methanol concentration. The LCST values of PND-9.5 linear copolymer in simulated alcoholic beverage increase linearly with an increase in the methanol concentration and a decrease in the ethanol concentration. The LCST values of PND-9.5 linear copolymer in the simulated alcoholic beverage with 0, 10, 20, 30 and 38 vol% of methanol are respectively 12.3, 19.5, 25.4, 29.6 and 34.1 °C, which increase averagely by 5.6 °C when 10 vol% ethanol is replaced by methanol.

Fig. 5 Effect of methanol concentration on the transmittance change of PND-9.5 copolymer in simulated alcoholic beverage. (a) Temperature-dependent transmittance change of PND-9.5 copolymer in simulated alcoholic beverage with the total alcohol concentration of 38 vol% but different methanol concentration; (b) corresponding LCST change of PND-9.5 copolymer in simulated alcoholic beverage as the function of methanol concentration.

The optimum operation temperatures are selected by easily distinguishing the transmittance of simulated alcoholic beverage with adjacent methanol concentrations in the investigated range. For example, the maximum difference between the transmittance of the PND-9.5 linear copolymer contained water/ethanol solution without methanol and that of water/methanol/ethanol ternary solution with 10 vol% of methanol appears around 17 °C (11.7 and 91.4% respectively). Therefore, 17 °C is selected as the optimum operation temperature. According to this principle, 17, 23, 28 and 31 °C are selected as optimum operation temperatures to investigate the optical transmittance of simulated alcoholic beverage with 10, 20, 30, 38 vol% of methanol. The quantitative results of the transmittances and the corresponding optical photographs of solutions at the optimum operation temperatures and at room temperature are shown in Fig. 6. The transparency of the simulated alcoholic beverage is divided into three levels according to the transmittance values. The transmittances in the range of 100–80%, 80–20% and 20–0% are respectively defined as transparent, semi-transparent and cloudy (Fig. 6a). The simulated alcoholic beverage without methanol (water/ethanol binary solution) is totally cloudy at all the investigated operation temperatures ranging from 17 to 31 °C. However, simulated alcoholic beverage with 38 vol% of methanol (water/methanol binary solution) is totally transparent at all the investigated operation temperatures. At each operation temperature, there is the critical methanol concentration below which the solution is cloudy, while above which the solution becomes transparent. For example, at 17 °C, the simulated alcoholic beverage without methanol (water/ethanol binary solution) is cloudy, while it turns transparent when 10 vol% ethanol is replaced by methanol. Due to the sharp increase of transmittance from 11.7 to 91.4%, the transmittance change can be easily recognized visually (Fig. 6b). Therefore, it is easy to distinguish whether the methanol concentration is lower or higher than the critical methanol concentration of 10 vol% at 17 °C. Similarly, methanol concentration below and above 20 vol% can be detected at the room temperature 25 °C. To distinguish higher methanol concentration, it is necessary to choose higher operation temperatures. Fig. 6 provide a database to figure out the methanol concentration range in the simulated alcoholic beverage of 38 vol%. The methanol concentration of alcoholic beverage of 38 vol% can be easily determined by colorimetric analysis at appropriate operation temperatures.

Fig. 6 Transmittance change of PND-9.5 copolymers in simulated alcoholic beverage with the total alcohol concentration of 38 vol% as the function of temperature and methanol concentration. (a) A quantitative result of transmittance change, which is categoried into three levels that respectively represented by the following symbols: ●0–20%; [circle shaded right slant]20–80% and ○80–100%; (b) optical photographs of the corresponding solutions for visual inspection.

As mentioned in the Introduction section, the minimum toxicity dose of methanol in the adults is only several milliliters. Detection of small amount of methanol in the alcoholic beverages is more meaningful from the applications' point of view. The temperature-dependent transmittance change and the corresponding LCST change as methanol concentration varying from 0 to 10 vol% are measured by using poly(NIPAM-co-DMAA) linear copolymers as indicators (Fig. 7 and 8). Although PND-9.5 linear copolymer can be used as the indicator to distinguish the methanol concentration at 17 °C, it fails at 25 °C because nearly no difference is observed between the transmittances of simulated alcoholic beverages with 0 and 10 vol% of methanol (Fig. 7a). The LCST values of PND-9.5 copolymer in the simulated alcoholic beverages with 0 and 10 vol% of methanol are 12.3 and 19.5 °C, which are both much lower than 25 °C. Thus, both of the simulated alcoholic beverages are cloudy at 25 °C. In order to obtain the methanol concentration between 0 and 10 vol% at room temperature, the PND-12.4 linear copolymer that has higher content of hydrophilic DMAA is chosen as the indicator. Fig. 7b shows that the LCST values of PND-12.4 linear copolymer in the simulated alcoholic beverages with 0 and 10 vol% of methanol increase to 20.0 and 27.8 °C respectively. At 25 °C, the transmittances of simulated alcoholic beverages with 0 and 10 vol% of methanol are respectively 10.9 and 85.6%. The large difference in the transmittances ensures easy detection of small amount of methanol in the simulated alcoholic beverage. Therefore, PND-12.4 linear copolymer is used in the subsequent measurements.

Fig. 7 Methanol-concentration-dependent and temperature-dependent transmittance change of PND-9.5 (a) and PND-12.4 (b) copolymers in simulated alcoholic beverage with the total alcohol concentration of 38 vol%, as well as the isothermal transmittance differences at 25 °C (on the dotted line).

Fig. 8 Detection of low concentration of methanol in simulated alcoholic beverage. (a) Methanol-concentration-dependent and temperature-dependent transmittance change of PND-12.4 copolymer in simulated alcoholic beverage with the total alcohol concentration of 38 vol%; (b) isothermal transmittance change of PND-12.4 in response to methanol concentration ranging from 0–10 vol% in simulated alcoholic beverage at 25 °C; (c) a quantitative result of transmittance change represented by the following symbols: ●0–20%; [circle shaded right slant]20–80% and ○80–100%; (d) optical photographs of methanol-induced transmittance changes in simulated alcoholic beverage with methanol concentration of 0 and 2.5 vol% at 25 °C.

A temperature-dependent transmittance change of the simulated alcoholic beverages with methanol concentrations of 0–10 vol% containing PND-12.4 linear copolymer as indicator is measured. The LCST values of PND-12.4 copolymers in the simulated alcoholic beverages with 0, 2.5, 5, 7.5 and 10 vol% of methanol are respectively 20.9, 23.7, 26.8, 28.0 and 28.9 °C (Fig. 8a). When per 2.5 vol% ethanol is replaced by methanol, the LCST values of PND-12.4 copolymers and optical transmittances of beverage samples containing PND-12.4 at 25 °C increase by approximately 2 °C and 20% (Fig. 8). The difference in the transmittances of about 20% is large enough to easily distinguish the two beverage samples by observing the transparency.

Fig. 8c shows the quantitative results of optical transmittance of the PND-12.4-contained simulated alcoholic beverages with methanol concentrations of 0–10 vol%. At 15 and 35 °C, the beverage samples are respectively categorized as transparent and cloudy according to the transmittance values, no matter how large the methanol concentration is (Fig. 8c). At 20, 25 and 30 °C, however, the cloudy beverage samples become transparent at the critical methanol concentrations. The critical methanol concentration increases with the increase of operation temperature. When 2.5 vol% of ethanol is replaced by methanol in the water/ethanol binary solution, the simulated alcoholic beverage containing PND-12.4 copolymer becomes semi-transparent from cloudy at 25 °C (Fig. 8c). The optical images of simulated alcoholic beverages containing 0 and 2.5 vol% of methanol at 25 °C confirm the difference in the transparency can be distinguished easily (Fig. 8d). The differences in LCST values and transmittances of simulated alcoholic beverages with 0 and 2.5 vol% of methanol are respectively 2.8 °C and 18.7%. The results show that the methanol concentration as low as 2.5 vol% in the simulated alcoholic beverage of 38 vol% can be detected simply and easily by observing the transparency of the solution.

At a certain operation temperature, the range of methanol concentration in the alcoholic beverage can be figured out by observing the transparency of the solution with poly(NIPAM-co-DMAA) linear copolymer as an indicator. The higher the methanol concentration in the alcoholic beverage is, the more transparent the solution exhibits. The minimum of methanol concentration can be detected by this colorimetric method is as low as 2.5 vol%. The presented method for methanol detection in alcoholic beverages by using poly(NIPAM-co-DMAA) linear copolymer as indicator is quite simple and low-cost, which also provides valuable material candidate for further designing simple and portable detection tools for general population especially in developing countries.

Conclusions

A novel, simple, visual, and low-cost method for quantitative methanol detection, which is carried out by using poly(NIPAM-co-DMAA) linear copolymer as indicator, has been successfully developed in this study. The differential alcohol-responsive characteristics of poly(NIPAM-co-DMAA) linear copolymers in response to ethanol and methanol in water/methanol/ethanol ternary solutions are systematically investigated. In water/methanol/ethanol ternary solutions with the total alcohol concentrations ranging from 10 to 40 vol%, the LCST values of PND-6.3 linear copolymers increase linearly with increasing the methanol concentration (C_M) and with decreasing the ethanol concentration (C_E) at a certain total alcohol concentration. The LCST values of the PND-6.3 linear copolymers increase averagely by 5.9 °C when 10 vol% ethanol is replaced by methanol in the water/methanol/ethanol ternary solution with the total alcohol concentration of 40 vol%. The LCST values of poly(NIPAM-co-DMAA) linear copolymers can be adjusted by varying the content of hydrophilic monomer DMAA, and thus the optimum operation temperature for methanol detection can be adjusted to around the room temperature, which makes the detection more easy and convenient. The simulated alcoholic beverage with total alcohol concentration of 38 vol% containing poly(NIPAM-co-DMAA) linear copolymer as indicator displays a gradient change in transmittance with the variation in temperature and methanol concentration. The methanol concentration is simply detected by observing the transmittance change of beverage samples containing poly(NIPAM-co-DMAA) linear copolymers at a certain operation temperature. The minimum of methanol concentration can be detected by using PND-12.4 linear copolymer as indicator at room temperature is as low as 2.5 vol%. Since the proposed method is quite simple and low-cost, it is highly promising to serve as new candidate tool for methanol detection for general population in developing countries and areas. Furthermore, such a differential alcohol-responsive system is also highly potential for further development of new detection or control systems, which is highly attractive for various applications in fermentation engineering, fine chemical engineering, food industry, and so on. Moreover, for the convenient operations, the poly(NIPAM-co-DMAA) copolymers can also serve as polymer brushes or smart surfaces that are capable to exhibit differential-alcohol-induced transmittance or wettability change to detect alcoholic products in daily life.

Acknowledgements

The authors gratefully acknowledge support from the Program for Changjiang Scholars and Innovative Research Team in University (IRT1163), and National Natural Science Foundation of China (21276162).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR spectra and GPC results of poly(NIPAM-co-DMAA) linear copolymers with different contents of hydrophilic monomer DMAA, and the effects of poly(NIPAM-co-DMAA) copolymers concentration on the transmittance change of the alcohol solution. See DOI: 10.1039/c4ra10996a

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