Novel methods and devices including modified endoscopes that employ fiber optic technology and multiple extrinsic optical sensors for enablement of simultaneous determination of change rates of biological analytes in vivo with single-cell resolution. The devices employ dynamic oscillatory actuation to determine the gradient of analytes of interest along one or multiple directions with respect to the cell. The change rate or flux of the analyte of interest can be easily determined by applying the first Fick's law that relates the flux with the concentration gradient. The dual operation mode of the device markedly increases measurement sensitivity and accuracy.