## 3 Results and Discussion
### Full-scale SERS Mapping of Brain Tissues  
We further investigated the capability of BP-AuNSs for label-free imaging of mouse brain tissues. The normal Raman mapping with a lateral resolution of 200 \(\upmu\)m was performed under the Raman microscope in the Streamline mode at a 9 s exposure time. Raman image of the whole brain tissue was acquired by integrating the intensity from 620 to 1720 cm\({}^{-1}\), which spend about 4.5 h (~9.3 s per pixel). Despite taking such a long time, the imaging quality of the normal Raman image (Figure 3a, red picture) was still very poor. The contour of the image was blurred and some regions were not displayed. The raw Raman spectral lines extracted from five different tissue regions showed only a small amount of fingerprint information of brain tissue, with high autofluorescence background (Figure 3b). The quality of Raman image became better after AuNPs were deposited onto the mouse brain tissue, which might be attributed to the weakened autofluorescence background induced by the metal nanoparticles (Figure 3c). However, weak Raman signals and fuzzy boundary were still not improved. The full-scale SERS scanning of mouse brain tissue attached with BP-AuNSs exhibited the most unambiguous image. It could be noted that a very short integration time of 1 s (about 33 min for a whole image) was enough to accomplish the mapping, which was several times faster than the Raman imaging without metal nanostructures treatment. In addition, the SERS spectra extracted from five regions demonstrated intense and plentiful chemical component information of brain tissue (Figure 3d), indicating that the promising potential of BP-AuNSs for neuroscience research.  
The mean Raman spectra of brain tissues were calculated form more than 100 spectral lines (static scanning, 10 s integration time, central wavenumber: 1200 cm\({}^{-1}\)). As shown in Figure 4, the normal Raman spectrum displayed the typical molecular vibration signals of brain tissue, for example: 1004 cm\({}^{-1}\) (symmetric ring breathing mode of phenylalanine), 1130 cm\({}^{-1}\) (C-N stretch in proteins, C-C stretch in lipids), 1200-1310 cm\({}^{-1}\) (phenylalanine, tryptophan, amide III, CH\({}_{2}\)/CH\({}_{3}\) deformation), 1400-1500 cm\({}^{-1}\) (CH\({}_{2}\)/CH\({}_{3}\) deformation, COO- symmetrical stretching, adenine, guanine) and 1560-1700 cm\({}^{-1}\) (amide II, amide I, adenine, guanine, phenylalanine, tyrosine) [6]. Gold nanoparticles only had a slight effect on the brain tissue, whose SERS spectrum was essentially consistent with that without nanoparticles treatment. In contrast, the SERS signals of brain tissue were significantly enhanced by BP-AuNSs, which were about 25 times (integrated intensities from 620 to 1720 cm\({}^{-1}\)) stronger than the normal Raman signals. Moreover, some new peaks appeared in the BP-AuNSs-based  
Figure 2: Surface-enhanced Raman scattering (SERS) spectra of crystal violet (CV) (10 \(\upmu\)M, **(a)** and IR-780 (1 \(\upmu\)M)), **(b)** under 785 nm excitation. The spectral lines represented the normal Raman spectra of the two dyes at the concentration of 1 mM (1), the SERS spectra induced by citrate-stabilized AuNPs (2), and the SERS spectra induced by BP-AuNSs (3), respectively.  
SERS spectrum, such as 780-880 cm\({}^{-1}\) (DNA/RNA: backbone O-P-O stretching, tyrosine, proline, C-O-C stretching in carbohydrate), 964 cm\({}^{-1}\) (C-C stretching in protein) and 1060-1195 cm\({}^{-1}\) (C-N stretching in protein and lipid, C-C stretching in lipid, protein and carbohydrate, O-P-O stretching in DNA, thymine, guanine) [5,6].