Biomolecule immobilization has attracted the attention of various fields such as fine chemistry and biomedicine for their use in several applications such as wastewater, immunosensors, biofuels, et cetera. The performance of immobilized biomolecules depends on the substrate and the immobilization method utilized. Electrospun nanofibers act as an excellent substrate for immobilization due to their large surface area to volume ratio and interconnectivity. While biomolecules can be immobilized using adsorption and encapsulation, covalent immobilization offers a way to permanently fix the material to the fiber surface resulting in high efficiency, good specificity, and excellent stability. This review aims to highlight the various covalent immobilization techniques being utilized and their benefits and drawbacks. These methods typically fall into two categories: (1) direct immobilization and (2) use of crosslinkers. Direct immobilization techniques are usually simple and utilize the strong electrophilic functional groups on the nanofiber. While crosslinkers are used as an intermediary between the nanofiber substrate and the biomolecule, with some crosslinkers being present in the final product and others simply facilitating the reactions. We aim to provide an explanation of each immobilization technique, biomolecules commonly paired with said technique and the benefit of immobilization over the free biomolecule.

Nanomaterials are promising alternatives to traditional bulk crystals for nonlinear optics and energy conversion materials. In this study, we report the observation of saturable second harmonic generation and high-order (up to 10th) multiphoton photoluminescence in colloidal MoS2 nanoflakes prepared by redox exfoliation which generates polyoxometalate clusters. Fourier transform nonlinear optical spectroscopy enabled the resolution of these high-order signals through two-dimensional nonlinear excitation/detection correlation spectra. Complementary modeling of the nonlinear interferograms suggested mechanisms for the observed high-order signals, which involve saturation of transitions resonant with the harmonic energy and multiple competing orders of multiphoton absorption. The saturable second harmonic generation and high-order multiphoton photoluminescence, which were not observed in MoS2 prepared by chemical vapor deposition, are related to the formation of polyoxometalate clusters during the redox exfoliation process. Further studies demonstrated that polyoxometalates can induce high-order nonlinear optical effects in other colloidal semiconductors with resonances at the harmonic energy, suggesting a general route to drastically alter the nonlinear optical response of nanomaterials with molecular adsorbates. This response is due in part to the interfacial charge transfer between polyoxometalate species and the nanomaterial. Fourier transform nonlinear optical spectroscopy offers a route to resolving the effects of molecular adsorbates, which are obscured in traditional nonlinear optical measurements.

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