Fabrication of a colorimetric paper-based microfluidic sensor for residual poly-dadmac detection in water treatment
Authors:   , Magubane Sibongile Elizabeth 
Published: August 2021

Poly– diallyldimethylammonium chloride (poly-DADMAC) is an established coagulant in the treatment of drinking water. Reports have indicated that poly-DADMAC can degrade into a suspected carcinogenic form which is N-nitrosodimethylamine (NDMA). Consequently, water treatment plant operators are required to know the residual concentration of polyelectrolytes at various stages in the treatment process and the eventual quality of the treated water. Historical research has proven that, over the years, a number of methods such as extraction-spectrophotometry, fluorometry and tannic acid have been developed and implemented for the analysis of polymers in drinking water. However, they produced poor linearity, sensitivity and precision, high detection limits or produced false positives due to matrix effects. The laboratory method that has proven to be simple, affordable and accurate is the colloidal titration method. However, this method cannot be used at the plant for quick and accurate monitoring of poly-DADMAC. In this study, the aim was to fabricate a Lovibond portable colorimetric comparator device based on gold nanoparticles for colorimetric quantification and detection of poly-DADMAC in raw and treated potable water. The colorimetric disk and comparator was fabricated from 14 nm gold nanoparticles with the concentration of poly-DADMAC varying from 1 to 10 mg L-1. The addition of higher concentrations of poly-DADMAC resulted in the aggregation of gold nanoparticles with the colour changing from red to blue. The gold nanoparticles were prepared via the citrate reduction method. Characterisation of the gold nanoparticles was done by ultraviolet- visible (UV/VIS) spectrophotometry and transmission electron microscopy (TEM). The Lovibond comparator was fabricated with a colour filter disk for the screening of residual poly-DADMAC in raw and potable water. The colorimetric disk was printed on the plastic slide and inserted in the plastic compartment of the comparator. The Lovibond comparator was verified with raw and potable water samples from different sampling points in and around the Mhlathuze river area located in the province of KwaZulu-Natal, South Africa. Preliminary results showed that the developed colorimetric comparator device can visually detect poly-DADMAC concentrations lower than 1 mg L-1. The colour development was first developed on normal paper and then optimised by UV/VIS spectrophotometry. The method developed has a linear range from zero to 10 mg L-1 with the correlation coefficient of R=0.9954. The effectiveness of the device was investigated by doing a recovery study on a potable water sample. Potable water is water that is suitable for drinking. In this research, potable water refers to tap water. The potable water sample was spiked with 1 mg L-1 poly-DADMAC. This exercise was done three times. The acceptance criterion for recovery is 80 to 120%. The 3 recoveries that were obtained are 107.95, 91.26 and 100.3%. The average recovery was 99.84%. This shows that the proposed method can detect poly-DADMAC with the acceptable level of accuracy. One of the important parameters that a quality method must have is selectivity. This parameter shows that the method can accurately detect the analyte of interest in the midst of different matrices. This was done by analysing the raw water samples together with their treated samples. Physical-chemical parameters were also analysed to show the broader state of the samples. The poly-DADMAC results obtained from the UV/VIS spectrophotometer compared quite well with those obtained from using the Lovibond colorimetric filter. The limited observation of colours using our eyes is a major contributor of systematic errors during the application of colorimetric devices. Thus, such a limitation can be reduced by using CIELAB system. A gold nanoparticle-based colorimetric CIELAB system for detection of poly-DADMAC in potable and raw water was also demonstrated. The method is based on the application of a paper-based analytical device which is printed on the normal A4 white printing paper. Fully enclosed 6 X 9 hydrophobic wells were fabricated on this paper. This work provides a clear evidence of the application of CIELAB colour system, and thus, replacing the conventional spectrophotometric technique to quantify polymers. Results of this work showed that the intensity of the fabricated well is proportional to the concentration of the detected polymer. The change in colour (ΔE) was calculated for each fabricated well and clear evidence of the colour change was observed upon the variation of the polymer. Moreover to the application of ΔE, the chromaticity using CIEYxy was used to verify colour change, it was observed that they followed the expected shift from red to blue, symbolising aggregation due to Van Der Waal inter-particle attractions as a result of the addition of poly-DADMAC. The results of this experiment were validated using the spectrophotometric technique which further emphasised the appearance of the new surface Plasmon resonance peak formed at 610nm symbolising aggregation. Importantly, the intensity of the new Surface Plasmon Resonance (SPR) peak at 610 nm increased by increasing the concentration of poly-DADMAC. Comparison of the Lovibond and UV/VIS results showed that there was no significant difference between the two methods. This proved that the fabricated Lovibond colour comparator is capable of the detection of residual poly-DADMAC in water treatment. This therefore implies that plant operators can be able to detect poly-DADMAC at any stage during the water treatment process by using a rapid, user-friendly portable device.