  In situ measurement of analytes for in vivo or in vitro systems has been
challenging due to the bulky size of traditional analytical instruments. Also,
frequent in vitro concentration measurements rely on fluorescence-based methods
or direct slicing of the matrix for analyses. These traditional approaches
become unreliable if localized and in situ analyses are needed. In contrast,
for in situ and real-time analysis of target analytes, surface-engineered
optical fibers can be leveraged as a powerful miniaturized tool, which has
shown promise from bio to environmental studies. Herein, we demonstrate an
optical fiber functionalized with gold nanoparticles using a dip-coating
process to investigate the interaction of light with molecules at or near the
surface of the optical fiber. Localized surface plasmon resonance from the
light-matter interaction enables the detection of minute changes in the
refractive index of the surrounding medium. We used this principle to assess
the in situ molecular distribution of a synthetic drug (methylene blue) in an
in vitro matrix (agarose gel) having varying concentrations. Leveraging the
probed Z-height in diffused analytes, combined with its in silico data, our
platform shows the feasibility of a simple optofluidic tool. Such
straightforward in situ measurements of analytes with optical fiber hold
potential for real-time molecular diffusion and molecular perturbation analyses
relevant to biomedical and clinical studies.
