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A biosensor is an analytical method that uses an active biological component that is integrated with electronic equipment to determine the analyte concentration. One of the biosensor methods widely developed is colorimetric-based detection. The purpose of this research was to study the potential of the use of purple spinach (Alternanthera brasiliana) leaf extract as a urea biosensor indicator. The research was performed including the leaf extraction, characterization, and application as a urea biosensor indicator. The results showed that the optimum purple spinach leaf extraction using Ethanol-HCl 85:15 (v/v) with an absorbance of 0.802 at a maximum wavelength of 531 nm. Furthermore, the leaf extract showed high stability in acid, low temperature (<4oC) storage, and the addition of reducing agent. The purple spinach leaf as an indicator of urea biosensor showed a linear correlation between the color change and urea concentration, with the regression line of y=-0.0007x+0.6397 and R2 of 0.961. [ABSTRACT FROM AUTHOR]

Large surface area of porous structure cryogel showed an excellent matrix for enzyme immobilization, especially in the biosensor development. A chitosan cryogel beads was prepare by cross-linking chitosan with sodium tripolyphosphate at subzero temperature. This chitosan cryogel beads was then used to immobilize glucose oxidase for glucose biosensor fabrication. The biosensor was simple design and operation, using a micropipette tip to hold the immobilized enzyme, where the reaction can be performed by suck-hold-release the analyte using micropipette. The detection was based on the reaction between hydrogen peroxide, enzymatic product, with titanium (IV) oxysulfate to produce color change, which finally recorded using commercial scanner. The digital image obtained was then analyze using freeware of ImageJ to get the relationship between color change and the analyte concentration. The result showed a linear response in the glucose detection of 1.0 to 5.0 and 10 mM, with a regression of y = 11.33x + 46.02 and R2 of 0.983. The great enzyme immobilization was showed in the fabricated biosensor with 12 times uninterrupted analysis without reducing significant responses. [ABSTRACT FROM AUTHOR]