Accurate measurement of microsphere mass and liquid properties (such as viscosity, density) is fundamental for elucidating the dynamic behavior of soft matter and colloidal systems, holding significant value in fields such as pharmaceutical processing, chemical detection, and microbiological research. However, existing high-precision techniques (such as suspended microchannel resonators) often fail to achieve simultaneous multi-parameter measurement, limiting comprehensive system characterization. To address this issue, this study aims to develop a high-throughput, high-precision method based on optical tweezers to synchronously measure microsphere mass, liquid viscosity, and liquid density, thereby meeting the interdisciplinary demands of fields like biomarker detection and soft material mechanics research.

A particle tracking system was constructed based on single-beam optical tweezers. The system employed a custom-built, high-stability, continuous-wave, single-frequency 671 nm laser, which was focused through an oil-immersion objective to form an optical trap at the center of the sample chamber for capturing microspheres. Particle displacement was tracked via a balanced photodetector using forward-scattered light interference. The theoretical model was based on the generalized Langevin equation, incorporating fluid memory effects. The velocity power spectral density was estimated using Welch’s method. Data were segmented (50% overlap), windowed using a Hamming window, and averaged to suppress spectral leakage and noise. Nonlinear least-squares fitting was applied to the power spectral density to simultaneously estimate parameters including microsphere density (ρ_p), radius (R), liquid density (ρ_f), and viscosity (η). Key parameters, such as segment length, were optimized to balance spectral resolution and statistical stability.

Experimental validation was performed using SiO₂ and polystyrene microspheres with diameters of 1 μm–5 μm in deionized water and dimethyl sulfoxide. The fitting results of the velocity power spectral density showed that for 1 μm SiO₂ microspheres, the parameters matched the nominal values (Fig.2). When the optimized segment length was set to 20000, the optimal accuracy was achieved. In water, microsphere mass was (1.295 ± 0.036) × 10⁻¹⁵ kg (coefficient of variation (CV) 2.8%), viscosity was (1.008 ± 0.025) mPa·s (CV 2.5%), and density was (1008.9 ± 13.0) kg/m³ (CV 1.3%). In dimethyl sulfoxide, viscosity was (1.983 ± 0.033) mPa·s (CV 1.7%), and density was (1104.6 ± 4.5) kg/m³ (CV 0.4%) (Fig.3). The method demonstrated robustness for different sizes and materials. For 1 μm ~ 5 μm SiO₂ microspheres, the mass errors were below 4.5%, with CVs less than 3%, among which the 3 μm microspheres showed the smallest CV of 0.84% (Fig.4). For 1 μm ~ 5 μm polystyrene microspheres, CVs were below 4.4% (Fig.5). The results confirmed that this method enabled high-precision simultaneous measurement of multiple parameters.

The simultaneous detection method proposed in this study achieves high-precision in situ synchronous measurement of microsphere mass and liquid properties by integrating optical tweezers tracking technology with advanced spectral analysis and optimized data processing. This method demonstrates excellent size applicability (1 μm ~ 5 μm), material applicability (SiO₂ and polystyrene), and robustness in different liquid environments. Optimization of parameters such as data segment length ensures statistical stability, and the theoretical model, which fully accounts for complex hydrodynamic effects, effectively reduces systematic bias. This study provides an effective technical foundation for future applications such as cytosol characterization and pharmaceutical fine processing, and demonstrates the potential for extending the method to nanoparticles or more complex fluid systems. This method holds significant importance for advancing interdisciplinary research in micro- and nano-scale multi-parameter metrology.