The sensors were constructed using B-GAL and CPRG based on either a linear single use lateral flow sensor or a multiplexed sensor design. In the linear design, a 0.5 cm 8 cm section of
Whatman no. 1 paper was inkjet printed with a substrate (CPRG) zone (denoted as CG zone) and a sensing (B-GAL) zone (denoted as color reaction or CR zone) using an underlayer of silica, the reagent (CPRG or B-GAL), and an overlayer of silica. These reagent zones were printed with a Fujifilm-Dimatix DP2800 printer using inks and settings as described elsewhere.31,32 To prevent leaching of colored product, a hydrophobic barrier (HB zone) was also printed on the top of the
sensing zone using either wax (by Wax printer, Xerox Phaser 8560N) or methyltrimethoxysilane (MTMS) (via piezoelectric inkjet printing) (Figure 1b). For the multiplexed sensor design, the hydrophobic barrier was first printed as a “flower” pattern using a Xerox Phaser 8560 wax printer (Figure 1c) to allow up to seven assays to be run simultaneously. The total sensor size was 3.5 cm 1.5 cm with a stem 2 cm long and 0.5 cm wide and a sensing zone measuring 1.5 cm 1.5 cm. CPRG was printed onto the arms of each assay zone, while B-GAL was printed in each of the circular regions. In some cases, the multiplex sensors used one assay channel for enzymatic sensing of heavy metals and had metalselective chromogenic reagents printed (as described above for CPRG but without the silica layers) into the circular region of other assay channels to allow for identification of metal ions present in a mixture. These included Zincon for detection of Hg(II), dimethylglyoxime for detection of Ni(II), sodium diethyldithiocarbamate for
detection of Cu(II), and 1,5-diphenylcarbazide for detection of Cr(VI), respectively. After printing, the sensor was allowed to dry for at least 1 h in air at room temperature.