## 3 Exogenous Reagents for PAI
Later, the team developed a covalent conjugating strategy for the synthesis of a MoS2-indocyanine green (ICG) hybrid, which significantly enhanced the PAI sensitivity compared to MoS2 nanosheets [63]. Moreover, the absorbing peak was redshifted from 675 nm to 800 nm, resulting in a greater penetration depth and lower background noise. In vivo PAM of orthotopic brain glioma demonstrated that MoS2-ICG has is capable of clearly identifying a tumour mass positioned 3.5 mm below the scalp, which is almost twice the depth achieved using S-MoS2 nanosheets. This is one of the greatest depths reported among all glioma PAM studies so far by using a nanoprobe in the NIR I spectral region (700-100 nm). Meanwhile, [PERSON] et al. [64] designed and synthesized conjugated polymer nanoparticles, which possesses strong optical absorption in the NIR II (100-1700 nm) 1064 nm wavelength. They used benzobisthiadiazole to copolymerize with Benzodithiophene to form poly(benzodithiophene-alt-benzobisthiadiazole) (P1) via Stille polymerization. The absorption peak at the 1064 nm wavelength of P1 nanoparticles (NPs) is suitable for PA generation when using common and low-priced Nd: YAG pulsed lasers. Moreover, the wavelength is in the NIR II range, which possesses less optical absorption and attenuation, but more penetration depth in skull and brain tissues compared with the light in NIR I, meaning it is better suited for deep imaging in the brain. In vivo PA imaging revealed that an orthotopic tumour range as deep as 3.4 mm can be clearly imaged in mice brains after an injection of P1 nanoparticles, and that 24 h after injection, the PA signal of the tumour had, in this instance, increased 94-fold. It reveals that P1 nanoparticles have a good biocompatibility, excellent photostability, high imaging contrast and signal/background ratio, passes the blood brain barrier (BBB) successfully and that it accumulates in tumours. Afterwards, the research group developed cyclo(Arg-Gly-Asp-D-Phe-Lys (mpa)) decorated P1 nanoparticles (P1 RGD NPs) [65], with an identical absorption spectra to P1 nanoparticles. P1 RGD NPs have a layer of c-RGD peptides covalently decorated on the P1 NPs' surface to enhance NP uptake inside a tumour. In combination with the 1064-nm laser illumination, the P1 RGD NPs enabled dual PA imaging and photothermal therapy in deep brain areas through scalp and skull, using the necessary optical power below the laser safety limit. Meanwhile, the group also developed a NIR-II fluorescent molecule with aggregation-induced-emission (AIE) characteristics for the imaging of orthotopic brain tumours. An encapsulation of the molecule in a polymer matrix yielded AIE dots, showing a very high quantum yield of 6.2% with a large absorptivity of 10.2 cm\({}^{-1}\)/gL\({}^{-1}\) at 740 nm and an emission maximum of almost 1000 nm [66]. These AIE dots can also be decorated with c-RGD to enable the specific and selective tumour uptake. Hence, this method presents great potential for precise dual NIR-I PAI and NIR-II fluorescence diagnostics of deep brain tumours through the scalp and skull. Recently, the group fabricated other conjugated polymer NPs, which showed an even higher absorptivity of 35.2 cm\({}^{-1}\)/gL\({}^{-1}\) at 1000 nm and an emission peak at 1156 nm [67]. These NPs were used to build a dual NIR-II PAI and fluorescence system that allowed for the mapping of small tumours measuring less than 2 mm at a depth of 2.4 mm beneath the scalp and skull. This method had a large signal-to-background ratio after BBB opening, and simultaneously resolved hemodynamics and cerebral vasculatures with a spatial resolution of 23 \(\upmu\)m at a brain depth of 600 \(\upmu\)m.  
For the imaging of deeper brain gliomas, [PERSON] et al. [68] designed and synthesized a mesoionic dye A1094, which was encapsulated in an Arg-Gly-Asp-modified hepatitis B virus core protein (A1094@RGD-HBc), with an aggregation-induced absorption enhancement (AIAE) mechanism in the NIR II window.