Single-molecule detection (SMD) provides ultrahigh sensitivity in biosensing and bioimaging, which is crucial for DNA and protein sequencing, early disease diagnose, drug detection, environment monitoring, food safety, and a deeper understanding of biological process, etc. Among various methods developed for SMD, fluorescence-based methods and transistor-based methods are outstanding due to their high sensitivity and versatility. However, fluorescence-based methods usually rely on labels, and advanced but bulky and expensive microscopes, restricting them to research use. For the label-free nanoscale field-effect transistors (FETs), detections of charged molecules of nanometres or larger in size are dubious due to the so-called Debye screening effect.

Here in this thesis, a novel optoelectronic device for molecular sensing is developed, combining the advantages of fluorescence-based methods and FET-based methods, while avoiding their disadvantages. Hydrogendoped amorphous InGaZnO thin film-based transistors (a-IGZO:H TFTs) are employed as phototransistors to detect photoluminescence signals from the upconversion nanoparticles (UCNPs) immobilized on top of the a-IGZO:H active layer. Molecular sensing is demonstrated using UCNP-based Förster resonance energy transfer (FRET) with streptavidin-biotin bonding pairs and small gold nanoparticles as quenchers. The integration of UCNPs and FRET with phototransistors simplifies the optics and gives a high signal-noise ratio (SNR). The integrated setup is promising to be developed into a portable and cost-effective biosensing platform with high sensitivity. To improve SNR, plasmonic nanostructures including aluminum nanohole arrays and gold nanorod dimers are simulated and fabricated to enhance the fluorescence intensities of fluorophores including single quantum dots, organic dyes on single extracellular vesicles, and single UCNPs. The plasmonic nanostructures for enhancing fluorescence intensities of single UCNPs can be integrated onto highly sensitive photodetectors in future studies to achieve SMD in electrolytes.