Herein, we developed a smartphone-based ratiometric resonance light scattering (RLS) device for portable field analysis and immunoassay. The RLS signal was detected by the camera in a smartphone controlled by a user-edited APP software. Except the smartphone, the total cost of the detection system itself is about 60 $, including a laser pointer and battery, a twin cuvette, a collective lens, and outside box. By using the RLS brightness ratio between the sample and reference channels in the twin cuvette as the signal indicator, the baseline drifts due to the variations in incident light intensity, ambient temperature, and shooting parameters of camera, can be eliminated efficiently. A new brightness indictor, (R - R-0)/(R-max - R), was proposed to linearize the calibration curve. Where R and R-0 are the rightness ratio in sample and blank, R-max is the maximum averaged brightness ratio, respectively. The applicability of the portable RLS device was demonstrated by the field analysis for Pb2+ in environmental water samples, using [CTA](2)PbI4 ] (CAT = cetyltrimethyl ammonium) as the luminophore. The limit of detection for Pb2+ is 0.7 mu gL(-1). By using PbS nanoparticles conjugated second antibody as the signal tag, the RLS method was applied in immunoassay to determine alpha fetoprotein with the limit of detection of 0.1 pg mL(-1). This smartphone-based ratiometric RLS device offers the advantages of good portability, low cost, high sensitivity and reliability.
