<p>Rapid, species-level identification of harmful algal blooms (HAB) remains constrained by laboratory-based molecular workflows and bulky fluorescence instrumentation. We report an integrated portable optoelectronic biosensing system for post-extraction quantification of <i>Heterosigma akashiwo</i> 18S rDNA. The key innovation is not the GO-based recognition chemistry itself, but the analytical signal-reading and conversion strategy: GO-regulated fluorescence recovery is captured by a sealed lens-filter-PIN photodiode module and converted through a resistor-based current-to-voltage circuit into an amplified photovoltage output. FITC-labelled ssDNA probes are quenched by GO nanosheets and recover fluorescence upon target hybridization. The recovered fluorescence is excited by a fixed-wavelength 488&#xa0;nm laser, filtered at 520 ± 10&#xa0;nm, collected by a sealed optical detection head, converted from photocurrent to photovoltage across a 10 kΩ sense resistor, amplified 500-fold, and digitized for quantitative readout. This voltage-mode architecture eliminates reliance on benchtop fluorimeters, microscope-camera assemblies, or picoampere-level current instrumentation, thereby improving the portability and practical usability of GO-based fluorescence assays. Using a 12-well double-layer microfluidic cartridge and a modified Stern-Volmer linearization strategy, the system quantified target DNA over 10^− 4-10⁵ pM with a limit of detection of 79.90 aM. After extracted DNA was introduced into the cartridge, the assay delivered results within 30&#xa0;min. Specificity was verified using one- and two-base mismatch sequences and non-complementary DNA from co-occurring HAB species. Mixed-sample tests at both gene and cell levels further demonstrated sequence-specific quantification. This work advances GO-regulated fluorescence HAB sensing toward a portable post-extraction voltage-output platform for rapid microalgal monitoring.</p> Graphical Abstract <p></p>