## 3 Results and Discussion
On the other hand, when Tyr crystals were photoexcited (Figure 2f), the same amounts of [Tyr\(+\) H]\({}^{+}\) and [Tyr\(-\)H]\({}^{-}\) ions were produced. Therefore, charge neutrality of the system was kept even after the photoexcitation and it was assumed that the buffer action of a Tyr molecule unlikely occurred, yielding a moderately intense peak (Figure 2f). Therefore, the concentration of the CdTe matrix was changed to 4 mg/mL in order to increase the number of electrons that could react with Tyr, and MALDI MS of Tyr was re-examined. The result is shown in Figure 2g. The peak intensity of [Tyr\(-\)H]\({}^{-}\) was increased compared with that in Figure 2e, although the intensities of the other peaks were also increased. Therefore, it might be possible to consider that the deprotonation of Tyr was promoted as a large number of electrons that could overcome neutralization were generated from CdTe nanoparticles at 4 mg/mL. From the above results, it was clarified that CdTe nanoparticles were applicable to the ionization of compounds containing carboxyl groups. The mechanism of electron ejection from CdTe in the  
Figure 2: Mass spectra of glucuronic acid (Glu) (**a**) with CdTe nanoparticles as the matrix and (**b**) without matrix. Mass spectra of ibuprofen (Ibu) (**c**) with CdTe and (**d**) without matrix. Mass spectra of tyrosine (Tyr) (**e**) with CdTe, (**f**) without matrix, and (**g**) with CdTe (4 mg/mL). ionization of fatty acids was previously studied by us [14]. An electron ejected by Auger recombination after the photoexcitation reacted with a carboxyl group in fatty acids, and the deprotonation reaction was promoted. Auger recombination is not unique to CdTe; this process occurs in other semiconductors as well. Therefore, another semiconductor nanoparticle, CuO, was used for the measurement of Tyr and stearic acid (Ste). Figure 3a shows the mass spectrum of Tyr measured with CuO at the concentration of 4 mg/mL. The peak of [Tyr\(-\)H]\({}^{-}\) was observed at \(m/z=180\). It was suggested that CuO had the same ability to deprotonate Tyr as CdTe by using an electron ejected after the photoexcitation. In addition, the peak intensity showed an almost 6.3-fold increase compared with that in Figure 2g. To understand the peak intensity change, a comparison of the zeta potential values of CuO and CdTe particles might be useful. It has been reported that the zeta potential usually changes with pH and that of CuO is approximately 25 mV around pH 7 [15; 16; 17] (sample preparation condition in this paper). CdTe nanoparticles used in this experiment were produced by laser ablation in water (without any stabilizer) and the CdTe nanoparticles easily aggregated in colloidal solution. Based on this fact, it was expected that the zeta potential of CdTe would be very small (approximately \(\pm 0\) mV). Therefore, the CuO surface was more positive than CdTe and it was assumed that CuO would attract lone-pair electrons from the oxygen or nitrogen atom in Tyr. Tyr could exist very near CuO so that electrons ejected from CuO would react with Tyr effectively, leading to the high peak intensity of [Tyr\(-\)H]\({}^{-}\). Then, Ste was subjected to MALDI MS using CuO nanoparticles. Figure 3b shows the mass spectrum of Ste; the peak of deprotonated Ste, [Ste\(-\)H]\({}^{-}\), was observed at \(m/z=238\). It was clarified that CuO had the ability to ionize fatty acids as well as CdTe. Measurements were also carried out for Ibu and Glu by using CuO, and spectra were shown in Figure A1 (Supplementary Materials). As the next step, the ionization of a rather high molecular weight compound, angiotensin II (AngII), was examined by using CdTe nanoparticles (4 mg/mL). The mass spectrum is shown in Figure 4a. Although the peak intensity of deprotonated AngII ([AngII\(-\)H]\({}^{-}\)) was very low and many peaks originating in other species were observed, it was understood that CdTe could be a matrix for peptides as well.