Photodetectors play a critical role in biomedical sensing, enabling precise and non-invasive measurements of physiological parameters. Their applications span pulse oximetry, fluorescence detection, and biochemical sensing, among others. The demand for high sensitivity, miniaturization, and biocompatibility has driven advancements in materials, device architectures, and integration with lab-on-a-chip platforms.

Miniaturization is a key requirement for biomedical photodetectors, particularly for wearable and implantable applications. Silicon-based photodiodes have been widely used due to their mature fabrication processes and compatibility with integrated circuits. However, organic photodetectors (OPDs) and perovskite-based detectors offer advantages in flexibility and tunable spectral response, making them suitable for conformal skin-mounted sensors. For example, OPDs with a near-infrared (NIR) response have been integrated into wearable pulse oximeters, achieving a signal-to-noise ratio (SNR) exceeding 30 dB in real-time monitoring.

Another approach involves silicon photomultipliers (SiPMs), which provide single-photon sensitivity for low-light applications such as fluorescence detection. These devices are critical in lab-on-a-chip systems where miniaturized optical setups require high gain and low dark current. SiPMs with active areas below 1 mm² have demonstrated detection limits in the femtomolar range for fluorescent biomarkers, enabling early disease diagnostics.

Biocompatibility is essential for implantable photodetectors, particularly those used in continuous glucose monitoring or neural activity sensing. Materials such as silicon carbide (SiC) and diamond exhibit chemical inertness and minimal immune response, making them suitable for long-term implantation. Diamond-based photodetectors, for instance, have shown stable operation in physiological environments for over six months without degradation. Additionally, encapsulation strategies using polymers like parylene-C or polydimethylsiloxane (PDMS) enhance biocompatibility while maintaining optical transparency.

Integration with lab-on-a-chip devices requires photodetectors to be co-fabricated with microfluidic channels and optical waveguides. Heterogeneous integration techniques, such as flip-chip bonding, enable the assembly of III-V photodiodes with silicon microfluidics for high-throughput biosensing. In fluorescence-based assays, this integration reduces optical losses and improves detection efficiency. For example, gallium arsenide (GaAs) photodiodes integrated into microfluidic chips have achieved a limit of detection (LOD) of 10 pM for DNA hybridization assays.

The spectral response of photodetectors must align with biomedical applications. Pulse oximetry relies on differential absorption of red (660 nm) and near-infrared (940 nm) light by hemoglobin. Silicon photodiodes with tailored antireflection coatings can achieve quantum efficiencies exceeding 80% at these wavelengths. For fluorescence detection, UV-sensitive photodetectors based on aluminum gallium nitride (AlGaN) or zinc oxide (ZnO) are employed due to their wide bandgap and low autofluorescence.

Noise reduction is critical for high-sensitivity detection. Active quenching circuits and lock-in amplification techniques are often employed to suppress ambient noise in wearable sensors. In implantable devices, shielding and differential readout configurations minimize electromagnetic interference from surrounding tissues. For instance, shielded SiPMs in implantable fluorescence sensors have demonstrated a noise-equivalent power (NEP) below 1 fW/√Hz, enabling single-molecule detection in vivo.

Emerging trends include the use of nanostructured photodetectors to enhance light absorption and reduce power consumption. Plasmonic nanostructures, such as gold nanoparticles, can locally enhance electric fields, improving the responsivity of organic photodetectors by up to 300%. Similarly, nanowire-based photodetectors offer high gain-bandwidth products, making them suitable for high-speed biosensing applications.

The future of biomedical photodetectors lies in multifunctional integration, where sensing, data processing, and wireless communication are combined on a single chip. Advances in flexible electronics and energy-efficient designs will further enable continuous, real-time monitoring in clinical and point-of-care settings.

In summary, photodetectors for biomedical sensing are evolving toward miniaturized, biocompatible, and highly integrated systems. Innovations in materials, device architectures, and noise suppression techniques are expanding their applications in diagnostics and personalized medicine. The convergence of photonics, microfluidics, and flexible electronics will continue to drive progress in this field.