Sensitive chlorine gas detection at the sub-ppm level using amphiphilic co-networks as highly convenient matrices

APCNs are copolymeric two-component networks consisting of hydrophilic and hydrophobic phases. These contrary phases build covalently co-connected networks and form nanophase-separated areas [Fig.1]. Silicon-hydrogel-APCNs are  i.a. used for long term contact lenses and drug delivery systems. Another innovative application of these amphiphilic networks includes the use as optochemical sensors.

Figure 1.  AFM phase mode image of the surface of PHEA-l-PDMS (25:75), dark=PDMS

We present a new optical polymer thin film sensor for high sensitivity and rapid response gas detection. The sensor consists of an amphiphilic co-network (APCN) in which the indicator has been embedded.

Due to the unique properties of APCNs, accurate gas detec-tion at the sub-ppm level is feasible within seconds.

APCNs with non-covalently immobilized (doped) indicators have outstanding properties for usage as optical sensors in gaseous and in liquid phases [1]. Due to the large interface between the phases – in bulk and surface – and the high gas permeability (silicon phase), a short response time results for detection in the sub-ppm range for gaseous analytes.

Here, we describe the first systematic determination of chlorine gas in the sub ppm-range within seconds, using o-tolidine as a suitable oxidation indicator.

The sensor films were prepared by copolymerization of 2-(trimethylsilyloxy)ethyl acrylate (TMSOEA) with the macromer α,ω-methacryloxymethyl-polydimethylsiloxan (MA-PDMS-MA, Wacker Silicones) [M_n=3200] on a previously methacrylate-modified glass slide, thickness limited by distance strips. Completed the reaction by removing of the protecting groups yields a 30 µm optical clear film of poly(2-hydoxyethyl acrylate-l-polydimethylsiloxan (PHEA-l-PDMS). Varying the monomer composite resulted in a 25:75 (PHEA:PDMS) (w/w) ratio for optimum gas detection sensivity [Fig. 2]. Finally loaded the thin film with the indicator in a methanolic o-tolidine solution and allowed to dry, leaving the indicator immobilized in the PHEA-phase.

Figure 2. Synthesis route for PHEA-l-PDMS

Chlorine gas measurements were taken in a modified UV-VIS spectrophotometer, with an adjusted flow of 1000 ml/min at several chlorine gas concentrations and variable humidity. Spectral development at the wavelength of 650 nm was detected and evaluated by correlating the change of absorbance in the starting phase with the chlorine gas concentration. (1)[Fig. 3].

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Figure 3.  Change of absorbance at 650 nm within the first 20 seconds. Reaction of chlorine with o-tolidine in PHEA-l- PDMS, 0% RH

The linear fit of the slope between two setting points of the detected absorbance leads to a maximum slope within the first seconds. As setting points we chose 0.004 and 0.008, similar to a change of transmission from 99.5% to 98.5%.
Detection times between 6 to 31 seconds for 1.0 to 0.2 ppm chlorine gas at 0% rH were achieved.
Chemical reaction of chlorine gas with o-tolidine is strongly affected by humidity which leads to decreasing sensitivity with increasing humidity [Fig. 4].

Figure 4. Measurement times for different chlorine gas concentrations at different humidity

 
Detecting at the OEL (occupational exposure limit) of 0.5 ppm chlorine gas (USA, EU) results in detection times bet-ween 8 – 30 sec, depending on humidity.

In accordance with lower sensitivity below 0.2 ppm of chlorine gas concentration and higher humidity, using a similar procedure of evaluation, analysis of the slope within the first 400 sec, yields a similiar result [Table 1].

Table 1.  Slope  values and measurement time

Compared to other polymeric optochemical sensors, we have increasing sensitivity of about 20%, while decreasing indicator concentration by one order of magnitude [2].
Even though APCN sensors have their greatest benefit for detecting hydrophilic substances in hydrophobic media and vice versa – e.g. enzymatic detection of hydroperoxides in n-heptane [3] - they have now proved their excellent gas sensing properties. Both required polymer conditions – high gas permeability and good indicator immobilization – are affected by the exceptional properties of APCNs.

References

[1]   M. Hanko, N. Bruns, S. Rentmeister, J. C. Tiller, J. Heinze, J. Anal. Chem. 78(18), 6376-6383, 2006

[2]   M. Ralfs, J. Heinze, Sens. Actuators B 44,  257-261, 1997

[3]   M. Hanko, N. Bruns, J. C. Tiller, J. Heinze, J. Anal. Bioanal. Chem. 386, 1273-1283, 2006