Ultra-low level detection of biomarker bilirubin by graphene quantum dots and bovine serum albumin enabled turn-on sensing

[Display omitted] •Detecting bilirubin at 0.15 nM level appears as the most sensitive available method.•Bilirubin may quench fluorescence of GQD through resonance energy transfer (FRET)•GQD may act as sensitive fluorescent ‘turn off-on’ probe for detection of bilirubin.•The proposed optical detection of bilirubin may help develop suitable sensor.•The work validates selectivity of GQDs and applicability to determine bilirubin in serum. It is known that concentration of unbound (free) bilirubin (BR) instead of the total bilirubin is the critical determinant of cellular uptake and toxicity of bilirubin. Thus, developing fast, effective and sensitive technique for detecting BR, a unique and important biomarker of liver dysfunction, has a long-standing demand. In this study, attempts were made to explore if biocompatible graphene quantum dots (GQDs) could be employed to probe BR at an ultra-low level in aqueous environment through optical techniques. Thus, understanding the possible interactions involved between GQDs and BR is important and necessary to develop protocol for probing BR. The detailed spectral analysis shows that BR molecule quenches the fluorescence of GQDs which may be attributed to a possible fluorescence resonance energy transfer (FRET) involved between the excited state of GQDs and BR. The micro-calorimetric and circular dichroic (CD) measurements have provided binding and structural information in the process of interactions. It is established that with the help of synthesized GQDs, BR can be detected as low as 0.15 nM selectively and this appears to be the most sensitive amongst other available methods. Interestingly, it is observed that the addition of bovine serum albumin (BSA) facilitates the recovery of the BR-induced loss in fluorescence intensity of GQDs. The adopted strategy demonstrates that GQD may act as a sensitive fluorescent “turn off–on” probe for detection of BR. The proposed optical method based protocol validated with serum samples isolated from blood for rapid detection of BR at ultra-low level may help in developing a suitable sensing device in future.