Phycocyanin (PC), a naturally fluorescent blue phycobiliprotein, is being explored in environmental analysis, food safety testing, immunoassays, and optical imaging. Researchers use it in two distinct ways: as a fluorescent sensing material that responds to another target, and as the target itself, where PC concentration serves as a marker of cyanobacteria. Keeping those roles separate makes the fast-growing literature easier to interpret.

1. From natural protein to engineered fluorescent probes

Many PC assays measure a change in fluorescence when the protein interacts with an analyte. Others pair PC with polymers or nanomaterials to improve signal strength, stability, or readout. The reported limits of detection (LODs) belong to particular methods and sample conditions; they should not be compared as if they were universal performance ratings.

• Conjugated-polymer micelles for PC monitoring. A cationic polymer, PFBT-MI, was placed in Pluronic F-127 micelles to support energy transfer to PC. The micellar system reported an LOD of 107.80 pM in water samples. Here PC is the analyte and a cyanobacteria-bloom biomarker—not a probe being used to detect an unrelated pollutant. The study also demonstrated a portable smartphone readout in lake and seawater samples. Read the polymer-micelle study.
• Carbon quantum dots for PFOS. A Spirulina-based study used commercial and extracted C-phycocyanin (C-PC), as well as carbon quantum dots (CQDs) prepared from extracted C-PC, to detect perfluorooctane sulfonate (PFOS). Fluorescence was quenched through an aggregation-associated response. Reported LODs were 1.48 nM for extracted C-PC and 1.73 nM for the CQDs, with spiked tap-water and bottled-water recovery experiments. Read the PFOS study.
• Upconversion nanoparticles for mercury and ciprofloxacin. A multicolor C-PC–UCNP sensor used an inner-filter effect to detect mercury ions (Hg²⁺) and ciprofloxacin (CIP). The study reported LODs of 0.05 μM for Hg²⁺ and 0.16 μM for CIP in aquatic-product testing. Read the UCNP sensor study.
• Protein engineering and hydrogel formats. A recombinant C-phycocyanin β-subunit (CpcB) system reported a fluorescence quantum yield of 0.38 and dual-mode mercury sensing. Separately, a phycocyanin–polyacrylamide hydrogel reported a 6.21 nM Hg²⁺ LOD and used smartphone images with a deep-learning model for visual quantification on fish surfaces. These are distinct proof-of-concept platforms. See the recombinant CpcB study and the hydrogel study.

2. Environmental monitoring and food-safety applications

Phycocyanin fluorescence is widely used as an indicator of cyanobacteria because PC is a characteristic pigment in many cyanobacterial species. A portable sensor can therefore help track changes in bloom-associated biomass. But PC is a proxy: its fluorescence per cell can vary with species, cell condition, temperature, and light, and a PC reading does not directly measure a toxin such as microcystin. The polymer-micelle study’s smartphone demonstration is an example of on-site PC measurement, not a toxin test.

For chemical pollutants, the PFOS studies illustrate two different materials: C-PC-derived carbon dots reached a reported 1.73 nM LOD, while a C-PC–titanium dioxide biohybrid reported 0.9375 ppb in aqueous testing. The upconversion platform targeted Hg²⁺ and CIP in aquatic products. These results are promising analytical demonstrations, but results from prepared or spiked samples do not by themselves establish routine field performance across water and food matrices. Read the C-PC–TiO₂ PFOS study.

One related alginate paper is sometimes described as a trace-mercury sensor. Its reported result was instead that alginate-immobilized C-PC removed more than 97% of Hg²⁺ from a test solution under specified pH and incubation conditions. That is a laboratory capture/remediation result, not evidence of a validated 2 ppb detection assay. Read the alginate immobilization study.

3. Biosensing, imaging, and immunoanalysis

PC–carbon-dot conjugates have also been investigated for peroxynitrite (ONOO⁻), a reactive nitrogen species. One ratiometric nanoprobe distinguished peroxynitrite from several other reactive oxygen species and reported a 0.5 μM LOD in solution, with recovery testing in spiked buffer. This is an analytical proof of concept; it does not yet demonstrate a clinical diagnostic or validated in-vivo assay. Read the PC–carbon-dot study.

Near-infrared imaging calls for a careful distinction between protein families. smURFP was engineered from the α-subunit of allophycocyanin, a related phycobiliprotein, and can bind biliverdin without a lyase. iRFP proteins, by contrast, come from bacterial phytochromes; one engineered iRFP variant carrying phycocyanobilin (PCB) showed more than four times the effective brightness of its biliverdin-bound form in cultured cells. These bilin-binding fluorescent proteins can support imaging in oxygen-limited settings, but they are not simply natural C-PC probes and should not be described as metal-ion sensors. See the smURFP engineering study and the iRFP–PCB study.

In food testing, a 2024 multiplex lateral-flow immunoassay used PC-labelled latex nanospheres to quantify aflatoxin B₁ (AFB₁), deoxynivalenol (DON), and zearalenone (ZEN) in grain and feed matrices. Smartphone-assisted visual reading took under 25 minutes in the reported setup. A separate recombinant streptavidin–CpcB fluorescent label was evaluated in an immunochromatographic assay for NT-proBNP, including comparisons with clinical serum measurements. These examples show how PC-derived fluorescence can serve as a signal label in immunoassays. Read the multiplex mycotoxin assay and the streptavidin–CpcB study.

What comes next?

Phycocyanin fluorescence research has moved from simple quenching tests toward engineered proteins, hybrid nanomaterials, portable readers, and image-analysis algorithms. The strongest near-term opportunities are environmental screening and food-safety assays, where researchers can compare sensor results with established analytical methods. Wider use will depend on reproducibility across batches, photostability, matrix-interference testing, calibration standards, and independent validation in real samples.

Takeaway: PC-based probes offer versatile fluorescence signals, and several studies report sensitive detection under defined laboratory conditions. The evidence remains method-specific. A low LOD alone does not establish a ready-to-use field test, toxin-risk assessment, or clinical diagnostic.