The speciation of selenium (Se) controls its fate and behavior, determining both its biological and environmental activities. However, in situ monitoring of SeIV presents a significant challenge due to its sensitivity to redox change. A novel diffusive gradients in thin films (DGT) technique containing mercapto-, amino-bifunctionalized SBA15 mesoporous silica nanoparticles was developed and evaluated in a series of laboratory and field deployment tests. The SBA-DGT exhibited a linear accumulation of SeIV ( r2 > 0.997) over a 72 h deployment, with negligible accumulation of SeVI(<5%). Consistent prediction of SeIV occurred within ionic strength and pH ranges of 0.1-200 mmol L-1 and 3.6-8, respectively. Limits of detection of the SBA-DGT were 0.03 μg SeIV L-1, which is suitable for natural waters. Moreover, the properties of the bifunctionalized SBA15 enable it to be fabricated within ultrathin (0.05 mm) gel layers for use in conjunction with O2 planar optode imaging. This new sandwich sensor technology with SBA-DGT was validated by mapping the two-dimensional distribution of SeIV and oxygen simultaneously in rice rhizospheres. This study shows that SBA-DGT provides a selective measurement of SeIV in situ, demonstrating its potential for both environmental monitoring and as a research tool for improving our understanding of Se biogeochemical processes.