Hemoglobin, an important indicator in routine blood tests, can reflect the ability of the body to produce red blood cells and assist in the diagnosis of a number of diseases, such as anemia, heart disease, and leukemia. Therefore, determination of hemoglobin content in human serum is an important element in clinical testing. Traditional hemoglobin detection techniques, such as colorimetric, electrochemical, fluorescence, and spectrophotometric methods, are typically associated with time-consuming, complex and cumbersome procedures; sample volume requirements; possibly high limit of detection; and possibly narrow dynamic ranges that limit further development of hemoglobin sensors to a certain extent. In this study, we report a novel optical fiber optofluidic laser hemoglobin sensor that exploits the “open-closed” ring mechanism of rhodamine spiro-ring derivatives for hemoglobin concentration measurement. This sensor achieves a lower limit of detection and wider dynamic range, while offering the advantages of simple operation and low sample consumption. We hope that the developed method will contribute to the design of new hemoglobin sensors with excellent performance and provide ideas for the design of optical fiber optofluidic laser biochemical sensors based on chromogenic reactions.

In this study, the “open-closed” ring mechanism of rhodamine B hydrazide induced by copper ions and the enzyme-like catalytic properties of hemoglobin were exploited. First, thin-walled hollow optical fibers with micron-sized wall thicknesses were prepared as optical microcavities for optical fiber optofluidic lasers using the corrosive effect of hydrofluoric acid. Subsequently, the ring-opening fluorescence product of rhodamine B hydrazide after redox hydrolysis was used as the gain medium that was passed into the prepared thin-walled hollow optical fiber. A radially emitted optical fiber optofluidic laser was achieved through the pump excitation of a pulsed laser. Next, the effects of the reaction time, concentration of rhodamine B hydrazide, and concentration of copper ions in the reaction solution on the optical fiber optofluidic laser output results were investigated and analyzed. Subsequently, the lasing threshold of the constructed optical fiber optofluidic laser was determined. Then, hemoglobin concentrations were measured under the optimized experimental conditions, along with fluorescence experiments for comparison with it.

The spectra of the collected whispering-gallery mode optical fiber optofluidic laser has sharp laser peaks in the wavelength range of 583-592 nm, with a full width at half maximum as low as 3.31 nm (Fig. 2). At 230 min, the optical fiber optofluidic laser appears, and the laser intensity increases rapidly in the time range of 230-325 min and subsequently reaches stability (Fig. 2). The results of the rhodamine B hydrazide concentration measurements show that rhodamine B hydrazide can increase the concentration of the fluorescent products of the ring-opening reaction and enhance the output laser intensity within a certain concentration range (Fig. 3). The results of the copper ion concentration measurements show that copper ions can increase the concentration of the fluorescent products of the ring-opening reaction and enhance the output laser intensity within a certain concentration range (Fig. 4). The lasing threshold test results show that the lasing threshold of the designed optical fiber optofluidic laser is approximately 9.74 μJ. When the pump energy exceeds the lasing threshold, the laser intensity increases linearly with the pump energy (Fig. 5). The performance test results of the hemoglobin sensor demonstrate that the designed sensor has a dynamic range of four orders of magnitude and a limit of detection of approximately 0.56 pmol/L (Fig. 6).

In this study, an optical fiber optofluidic laser biochemical sensor for hemoglobin detection is proposed and demonstrated using the “open-closed” ring mechanism of rhodamine B hydrazide, a rhodamine spiro-ring derivative. Using the prepared thin-walled hollow optical fiber as a whispering-gallery-mode optical microcavity, the highly fluorescent product rhodamine B, produced by the ring-opening reaction of rhodamine B hydrazide induced by copper ions, is used as a gain medium to achieve radially emitting optical fiber optofluidic laser. The enzyme-like catalytic properties of hemoglobin facilitate the ring-opening reaction of rhodamine B hydrazide that has been exploited in the design of hemoglobin sensors. The effects of the reaction time, concentration of rhodamine B hydrazide, and concentration of copper ions in the mixed solution on the laser output results are investigated, and the threshold of the optical fiber optofluidic laser is tested. By optimizing the experimental conditions through result analysis, the designed hemoglobin sensor offers a dynamic range of four orders of magnitude and a limit of detection on the pmol/L scale.