## 3 Results and Discussion
### Practical Assay Detection
#### 3.2.2 Colorimetric Measurement for Creatinine and Albumin Quantification  
The creatinine assay kit (BioChain Institute Inc., Newark, CA, USA) based on the Jaffe method [21] was used to demonstrate the quantification of the creatinine concentration. The yellow picate solution turned red upon forming a picrate-creatinine complex in an alkaline medium. Samples with seven concentrations of creatinine (500, 400, 300, 200, 150, 100, and 50 mg/L) prepared by diluting the creatinine standard provided by the manufacturer in deionized water and one blank sample (0 mg/L creatinine, deionized water only) were used to generate the standard curve and evaluate the GGP-GMR/CCD detection system. The working reagent containing picrate, which was also provided by the manufacturer, was added to each concentration to form the complex. An LED with a wavelength of 535 nm was used to illuminate the liquid sample. The transmitted intensity and spectrum were recorded immediately after loading the working reagent (0 min) and after 5-min incubation. Figure 7a,b present the spectra recorded for the sample with 500 mg/L of creatinine by using the GGP-GMR/CCD system and spectrometer, respectively. The varying intensities between 0 and 5 min at 535 nm and the 2100 th pixel for different concentrations were used to construct the dose response curves for the GGP-GMR/CCD device and spectrometer (Figure 7c,d, respectively). The intensity variation increased with the creatinine concentration, with the largest intensity variation observed at the highest concentration of 500 mg/L. The correlation between the measurement results obtained with the spectrometer and GGP-GMR/CCD device by normalizing the intensity variation at each concentration to 500 mg/L is illustrated in Figure 7e. The slope and R2 values indicate a comprehensive agreement between the results obtained with the spectrometer and proposed device. The dose response curves displayed in Figure 7c,d indicate that the results obtained with the spectrometer and proposed system were highly close. Within the range of 50-500 mg/L, both GGP-GMR/CCD system and spectrometer exhibit a fairly linear response. The spectrometer had higher sensitivity and noise than the proposed system did, which resulted in the spectrometer having a marginally higher LOD. However, the LOD values obtained with both systems were higher the value suggested by the manufacturer, namely 1 mg/L. The main reason for this result is the continuous variation in intensity due to the persistent reaction between creatinine and the working reagent. To overcome this problem, the experiment must be executed in a highly proficient manner and the environmental lighting must be strictly controlled. Nevertheless, the location and intensity of the MICP obtained using the GGP-GMR/CCD system exhibit a suitable correlation with the peak wavelength and its intensity obtained using the spectrometer. Another example of a colorimetric assay is albumin detection based on its binding with bromocresol green. On the formation of the albumin-bromocresol green complex, the color of the assay solution changes from green to blue, and the peak absorbance is observed at 620 nm. The assay was fairly straightforward according to the manufacturer's manual. We simply mixed different concentrations of albumin in deionized water (5-50 mg/mL) and the bromocresol green reagent provided by the manufacturer in a 1:40 ratio in a cuvette and incubated the mixtures for 5 min. An LED with a wavelength of 625 nm was used to illuminate the liquid samples. Due to the high absorbance at 620 nm upon complex formation, the transmitted intensity decreased with an increase in the albumin concentration for the measurements obtained with the GGP-GMR/CCD system and spectrometer (Figure 8a,b, respectively). The peak wavelength was approximately 636 nm. By contrast, the MICP had pixel numbers of 2247-2251, which agrees well with the calibration diagram presented in Figure 1e.