Blood lactic acid concentration is an important indicator for physiological functions. To develop a rapid and sensitive
measurement technique for monitoring blood lactic acid may provide a useful tool in clinical diagnosis. We proposed to
develop a microdialysis surface-enhanced Raman spectroscopy (microdialysis-SERS) approach to filter/reduce
interference from other large metabolites in blood and enhance the detection sensitivity for blood lactic acid. In this
study, a microdialysis probe was constructed using 13 kDa cut-off dialysis membrane. The dialysate was mixed with 50
nm Ag colloidal nanoparticles automatically in a micro-fluid chamber for SERS detection under blood microdialysis of
Sprague-Dawley rat. The linear range of SERS-lactic acid measurement is 10<sup>-5</sup>~3x10<sup>-4</sup> M with R<sup>2</sup> value of 0.99. The
optimal mixing flow rate of nanoparticles is 18 μl/min under microdialysis at constant flow rate (2 μl/min). Real time
lactic acid monitoring in vivo also has been demonstrated using microdialysis-SERS system.
Highly sensitive measurement of biomolecules is very important in clinical diagnosis and biomedical sensing. Spectroscopic methods have played important roles in biomedical sensing system developments. Recent development in surface enhanced Raman scattering (SERS) method has greatly enhanced the weak Raman signals of biomolecules and has provided great potentials for real time measurement of biomolecules of body fluid. In addition, Raman measurement has the advantage of not requiring extrinsic fluorescent marker for labeling purpose. In this study, we have pioneered in the development of SERS spectroscopic measurement technique for serum lactic acid, which is one of the most important metabolic parameter in blood. We have fabricated Ag colloidal nanoparticles to enhance the weak Raman signal of lactic acid in serum. The diameter of the Ag nanoparticle is 20 nm, the nanoparticles concentration is 10<sup>9</sup>particles/ml. We have observed the SERS characteristic peak of lactic acid at 1285~1480cm<sup>-1</sup> under 632.8 nm HeNe laser excitation. We have demonstrated the measurement of the lactic acid in filtered serum in the physiological concentration range 5x10<sup>-3</sup>~22x10<sup>-3</sup> mole/L, which is hundred times lower than the detectible range using traditional Raman approach. The serum samples with were measured in a specially designed reflector type sample holder to form a multiple reflection of excitation laser through the sample, between a reflector and a notch filter. In conclusion, this research demonstrates the feasibility of using Ag SERS technique for measuring the lactic acid at physical concentration and establishes the platform technique for human body fluid measurements.