Chemiresistive sensors have been widely used for gas sensing but recently few studies have reported their use in liquids. Unlike electrochemical sensors, these sensors are simple to fabricate and operate with a single sensing film electrodes with two electrical contacts for monitoring change in current under a small potential bias, without the need of reference electrode. Hydrogen peroxide (H2O2) is an intermediate molecule generated in numerous peroxidase assays. Therefore, it is widely used for detecting multiple biomolecules like glucose, galactose etc. H2O2 detection is commonly performed using colorimetric and electrochemical sensors. However, colorimetric sensors require chemicals while electrochemical sensors need reference electrode for reliable measurements .
Here, we demonstrated a reagent less H2O2 sensor fabricated using carbon nanotube (CNT) as the base substrate and an electropolymerized self assembled monolayer crystal violet as selective ligand. The functionalised 2D percolation network of CNT film transduces a selective response for H2O2. The chemiresistive sensor was fabricated using a low-cost xurography based fabrication process. The CNT dispersion was supplied by Nano-C, Inc. A diluted CNT dispersion (4% v/v) was prepared by sonicating the CNT ink in a mixture of methanol and water (1:1) for 15 minutes in a bath sonicator. The diluted CNT dispersion was drop casted on a frosted glass substrate and cured at 120 oC for 60 minutes. Patterned gold leaf was used as the contact electrodes and copper tape was used for the external electrode (Figure 1a). The sensor fabrication process is described in details elsewhere . The electrodes were functionalized with a self assembled monolayer by immersing them in 0.6 mM crystal violet prepared in 0.1 M phosphate buffer for 90 minutes. Then, the self assembled monolayer of crystal violet was electropolymerized using cyclic voltammetry from 0 to 2V and scan rate of 0.1 V/s.
The sensor response was tested in five different H2O2 concentrations (0.5 ppm, 5 ppm, 50 ppm, 500 ppm, and 1000 ppm). H2O2 solutions were prepared by diluting the stock solution in 39 mM acetate buffer similar to the buffer strength of blood. The sensor response for various H2O2 concentrations is shown in Figure 1b. The sensor response for each H2O2 solution was calculated with 39 mM acetate buffer as the baseline. Each bar graph represents the average of last-minute current measurements of a 30 minutes run. The sensor exhibited an increase in current value with increase in H2O2 concentration from 0.5 ppm to 1000 ppm. The crystal violet attached on the CNT film undergoes oxidation in presence of H2O2 concentration. The change in oxidation state of crystal violet resulted alterations in doping level of functionalized CNT network leading to a change in conductivity of the sensor. No significant interference was observed for common interferents like as glucose, galactose, urea, uric acid and gluconic acid. The sensor was also chemically reset using 0.1 M ascorbic acid by reducing the oxidized crystal violet.
We demonstrated that an electropolymerized self assembled monolayer of crystal violet attached to CNT can measure H2O2 concentration within a range of 0.5 ppm to 1000 ppm. In addition, the low-cost fabrication process and the peroxide sensor enables this technology to a wide range of peroxidase based enzymatic assays that are biological relevant both in medical diagnostics biological research.
 V. Patel, P. Kruse, P.R. Selvaganapathy, Solid State Sensors for Hydrogen Peroxide Detection, Biosensors. 11 (2020) 1–32. https://doi.org/10.3390/bios11010009.
 V. Patel, P. Kruse, P.R. Selvaganapathy, Flexible chemiresistive sensor with xurographically patterned gold leaf as contact electrodes for measuring free chlorine, (2021) 1–4. https://doi.org/10.1109/fleps51544.2021.9469787.