**4.1 Paper/strip**

Making use of paper for the spot analysis of copper can be observed way back to 1945 [83]. Lateral flow through capillary action is the basis for the development of test strips not only for HMIs but various analytes. Pioneering work was published by Whiteside's group in the year 2007. In which photolithography was used to pattern the paper for the bioanalysis on the spot [84]. Paper based analytical device (PAD) is modified with resorufin thionocarbonate-based dye and it can detect μM concentration Hg2+ ions by using buffer solution of pH 8 [85]. 3,3′,5,5′-tetramethylbenzidine (TMB) and gold nanoparticles were used to develop a PAD for semiquantitative analysis of Hg 2+ ions. Based on the enzyme like action of the gold and mercury nanoparticles TMB will give blue color [86]. In a similar work, platinum nanoparticles and TMB combination is used to detect Hg2+ ion using PAD. In this case decrease in the color intensity indicates the concentration of HMI. Enzymatic activity of the platinum nanoparticles to turn the TMB to blue is inhibited by the Hg2+ ions in this case [87]. Many such PAD based naked eye detection of HMIs can be found in literature [88–90]. Paper based electrochemical sensors (PESs) are coined as they inherit the simplicity and advantages of the PADs with better sensitivity. Whiteside's group developed PES by adopting the commercial electrochemical readers [91]. In case of PES electrodes are printed on a paper with carbon inks (working and counter electrodes), silver-silver chloride inks (reference electrode). These electrodes connected to a respective terminal of the electrochemical reader to measure a signal. Mariana Medina-Sánchez et al. developed PES for the quantification of Pb2+ and Cd2+ ion in a range 10 to 100 ppb. Developed sensor was reagent free and ecofriendly [92]. Two substrates i.e. plastic film and paper were used to coat graphene and polyaniline composite for the simultaneous detection of Zn2+, Cd2+, and Pb2+ ions [93]. Going one step ahead Poomrat Rattanarat et al. came up with a multi-layer-based device capable of producing both optical and electrochemical signal for iron, nickel, copper, chromium, lead and cadmium ions [94]. Iron, nickel, copper, chromium can be detected using optical signal whereas lead and cadmium ions are detected by electrochemical output as shown in **Figure 6**.

#### **Figure 6.**

*(A) Image of the multilayer paper-based device's color changing response for the given HMIs. (B) Flow diagram details the preparation, electrochemical, and colorimetric response of the multilayer device (reprinted with permission from [94] copyright 2014 American Chemical Society).*
