Ionization Time-of-Flight Mass

Article pubs.acs.org/ac

Negative Ion Laser Desorption/Ionization Time-of-Flight Mass Spectrometric Analysis of Small Molecules Using Graphitic Carbon Nitride Nanosheet Matrix Zian Lin,†,‡ Jiangnan Zheng,† Guo Lin,† Zhi Tang,‡ Xueqing Yang,‡ and Zongwei Cai*,‡ †

Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350116, China ‡ Partner State Key Laboratory of Environmental and Biological Analysis, Department of Chemistry, Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, Hong Kong, SAR, China S Supporting Information *

ABSTRACT: Ultrathin graphitic carbon nitride (g-C3N4) nanosheets served as a novel matrix for the detection of small molecules by negative ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was described for the first time. In comparison with conventional organic matrices and graphene matrix, the use of g-C3N4 nanosheet matrix showed free matrix background interference and increased signal intensity in the analysis of amino acids, nucleobases, peptides, bisphenols (BPs), and nitropolycyclic aromatic hydrocarbons (nitro-PAHs). A systematic comparison of g-C3N4 nanosheets with positive and negative ion modes revealed that mass spectra produced by g-C3N4 nanosheets in negative ion mode were featured by singly deprotonated ion without matrix interference, which was rather different from the complicated alkali metal complexes in positive ion mode. Good salt tolerance and reproducibility allowed the determination of 1nitropyrene (1-NP) in sewage, and its corresponding detection limit was lowered to 1 pmol. In addition, the ionization mechanism of the g-C3N4 nanosheets as matrix was also discussed. The work expands its application scope of g-C3N4 nanosheets and provides an alternative approach for small molecules.

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universal optical adsorption property. However, most of the carbon material matrices showed poor solubility and water dispersibility, which usually leads to the inhomogeneous crystallization with the analytes and, thus, decreases the sensitivity of SALDI MS and shot-to-shot reproducibility.23 On the other hand, although most of the carbon-based matrices used in SALDI MS were commonly based on positive ion mode owing to the universality and sensitivity of chemicals, the complicated multialkali metal ion adducts along with protonated ions in the positive ion spectra often made it difficult to identify.16,25 In contrast, the negative ion spectra can provide relatively clear background and is very easy to interpret due to the feature of singly deprotonated ion. The successful application of graphene matrix in negative ion mode has been made by Cai’s group in 2011,21 who also found that the graphene matrix exhibited high sensitivity in the analysis of small molecules. More recently, nitrogen-doped graphene, as one of modified carbon-based materials, has been developed as a matrix for negative ion SALDI MS. Due to the extraordinary electronic properties of nitrogen, the N-doped graphene matrix

atrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has been widely applied to the analysis of synthetic polymers and proteins over the past two decades.1,2 Although some remarkable achievements have been made, analysis of low-molecular-weight compounds (m/z < 500 Da) by MALDI-TOF MS using conventional organic matrices (e.g., α-cyano-4-hydroxycinnamic acid, CHCA) remains a great challenge due to the strong matrix-derived interference in the low mass region.3 Furthermore, the inhomogeneous co-crystallization of analytes with an organic matrix often requires sweet-spot searching, which results in low shot-to-shot reproducibility.4 To solve these problems, great efforts have been made recently. One strategy is to exploit new organic matrices with low background.5,6 Another one is the development of surfaceassisted laser desorption/ionization mass spectrometry (SALDI-MS)7 by using inorganic nanomaterials as matrices. At present, a diversity of inorganic nanomaterials including porous silicon,8,9 metal/metal oxide nanoparticles,10−13 inorganic carbon-based materials,7,14−16 and metal−organic frameworks17,18 have been introduced as SALDI matrices. In particular, using inorganic carbon-based nanomaterials like fullerene,19 graphite,15 carbon nanotubes,14,20 graphene,16,21,22 and carbon nanodots23,24 appears to be an attractive ionization method due to the merits of remarkable charge mobility and © XXXX American Chemical Society

Received: June 2, 2015 Accepted: July 14, 2015

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DOI: 10.1021/acs.analchem.5b02066 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

Figure 1. (A) Photograph of bulk g-C3N4 and suspension of ultrathin nanosheets. (B) TEM image of the g-C3N4 nanosheets. (C) AFM image of the g-C3N4 nanosheets. (D) The corresponding height image of two random nanosheets. (E) UV−visible spectrum of bulk g-C3N4 and nanosheets.

nitropyrene (1-NP) in sewage. In addition, the ionization mechanism of g-C3N4 nanosheets as matrix was also discussed in detail.

exhibited high LDI efficiency, and thus, strong MS signals were achieved in comparison with the graphene matrix.26 Nevertheless, the process for preparation of the N-doped graphene by a modified thermal annealing method is rather difficult to control. Therefore, the development of new matrices with easy preparation, good stability, and dispersibility, and high ionization efficiency is highly desirable. As the most stable carbon nitride allotrope, bulk graphitic carbon nitride (g-C3N4) has attracted increasing attention in recent years and has been widely used in photochemical catalysis.27,28 Different from bulk materials, g-C3N4 nanosheets of atomic-scale thickness and nanometer scale process a graphite-like lamellar structure and high surface area, which facilitates the charge transfer.29,30 Recently, g-C3N4 nanosheets have been widely applied in the field of sensing and bioimaging31−34 since the first report by Xie’s group.35 Significantly, g-C3N4 nanosheets can offer high adsorption capability in the UV/vis region due to its unique π-conjugated structure;35 it is believed that the g-C3N4 nanosheets can satisfy the criteria to serve as a SALDI matrix. However, to the best of our knowledge, any effort to evaluate g-C3N4 nanosheets as novel SALDI matrix has not been reported so far. In this paper, ultrathin g-C3N4 nanosheets were prepared through a facile liquid exfoliation approach and then for the first time introduced as a highly efficient matrix for MALDI-TOF MS. A variety of small molecules were selected as test compounds to evaluate the matrix performance. In comparison with organic matrices and graphene matrix, the g-C 3N4 nanosheet matrix in negative ion mode exhibited significant advantages, such as high sensitivity, free matrix background, good salt tolerance, and excellent reproducibility. These unusual features allowed it for quantitative analysis of 1-

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EXPERIMENTAL SECTION

Synthesis of Bulk g-C3N4. The bulk g-C3N4 was synthesized according to a previous report with minor modification.35 In brief, melamine was heated at 600 °C for 12 h with a ramp rate of 2 °C/min for both of heating and cooling processes. The obtained yellow powder was the bulk gC3N4. Preparation of Ultrathin g-C3N4 Nanosheets. Ultrathin g-C3N4 nanosheets can be obtained by a simple liquid-phase exfoliation of the bulk g-C3N4 in water.35 Briefly, 5 mg of bulk g-C3N4 powder was dispersed in 1 mL water, followed by 10 h of ultrasonication. Afterward, the formed dispersions was centrifuged at 13200 rmp in order to remove the unexfoliated g-C3N4 particles and then the supernatant was collected for further study. Sample Preparation for MALDI- or SALDI-TOF MS. CHCA (10 mg/mL) and 3-aminoquinoline (3-AQ, 10 mg/mL) were individually prepared according to our previous work.18 The g-C3N4 nanosheet solution (∼0.75 mg/mL) was vortexed for 30 s prior to use. As a control, graphene matrix was also prepared by adding 1 mg graphene nanopowder into 1 mL ethanol−aqueous solution (1:1 v/v) and sonicated for 5 min before use. A total of 1 μL of matrix solution was applied onto the sample plate and air-dried, followed by 1 μL analyte solution (0.5 μL for BPs), was pipetted on top of the matrix crystals. B


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Figure 2. Mass spectra of histidine by using (A) CHCA matrix in positive ion mode; (B) 3-AQ matrix in negative ion mode; (C) Graphene matrix in positive ion mode; (D) Graphene matrix in negative ion mode; (E) g-C3N4 nanosheet matrix in positive ion mode; and (F) g-C3N4 nanosheet matrix in negative ion mode. The concentration of His was set as 1 mM. Laser intensity: 60%. Red asterisk is background peaks of matrix unless otherwise noted.

Mass Spectrometry. MALDI-TOF MS experiments were performed on a Bruker Autoflex II mass spectrometer (Bruker Daltonics, Germany) equipped with a nitrogen laser operated at 337 nm and laser attenuator offset of ∼67% in positive and negative reflection mode. Other MS parameters were described in Supporting Information.

The uniformity of the g-C3N4 nanosheet matrix with the analyte solution was also examined and the results are shown in Figure S3 (Supporting Information). The heterogeneous cocrystallization of CHCA matrix with analytes was observed from Figure S3A (Supporting Information), which caused hot spots and made it difficult to find sweet spots. In contrast, the g-C3N4 nanosheet solution with analytes deposited on the sample plate resulted in coverage of the matrix spot densely and enormous number of homogeneous microcrystals were found (Figure S3B, Supporting Information). Obviously, the uniform distribution of the matrix/analyte mixture made it possible for improving shot-to-shot reproducibility and the accuracy of signal distribution. MALDI-TOF MS of Amino Acids. The LDI mass spectra of bulk g-C3N4 and its nanosheets were detected. As shown in Figure S4 (Supporting Information), a strong ion peak at m/z 200.01 with the interval of 25 mass unit was observed when direct ionization of the bulk g-C3N4. Obviously, the strong ion peak at m/z 200.01 was attributed to the deprotonated ion of melon (Figure S5, Supporting Information). Moreover, these oligomeric ion peaks with the same repeat number dominated the spectrum, thus causing significant background interference. Compared with the bulk g-C3N4, there were no matrix peaks appeared in the low mass range when directly ionizing g-C3N4 nanosheets, indicating that the g-C3N4 nanosheets is suitable for use as SALDI matrix in the analysis of small molecules. In the preliminary application test, His (MW = 155.15) was chosen as representative and then analyzed by MALDI- and SALDI-TOF MS using CHCA, 3-AQ, graphene, and g-C3N4 nanosheets as matrices in both ionization modes. With CHCA matrix, the His-related peaks, which were designed as [M + H]+, [M + Na]+, [M + K]+, [M + 2Na−H]+, [M + Na + K− H]+, and [M + 2K−H]+ ions, were obtained in positive ion spectrum (Figure 2A). However, the fragment peaks of CHCA matrix predominated the spectrum, and significantly suppressed the signals of His. Under negative ion mode, no signal of His

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RESULTS AND DISCUSSION Characterization of g-C3N4 Nanosheets. Figure 1A presented the photographs of the yellow bulk g-C3N4 powder and the dispersion of product obtained by ultrasonic treatment. It was observed that the suspension was nearly transparent, implying its ultrathin thickness. It should be noted that no aggregation was observed in the suspension, even standing for more than two months. The result indicated that the g-C3N4 nanosheet solution was very stable. In addition, the concentration of the exfoliated g-C3N4 nanosheets was estimated to be approximately 0.15 mg/mL according to the previous report.35 Figure 1B showed the TEM image of the exfoliated product and its diameter was approximately 200 nm. The AFM image of the g-C3N4 nanosheets confirmed that they were well-separated from each other (Figure 1C). Section analysis demonstrated that the thickness of the nanosheets was ∼2 nm (Figure 1D), indicating that the g-C3N4 nanosheets composed less than 12 C−N layers. XPS and FT-IR measurements were performed and the results demonstrated that the bulk g-C3N4 was mainly consisted of C and N elements (Figure S1−2, Supporting Information). As presented in Figure 1E, the absorption edge of the g-C3N4 nanosheets shows a blue shift compared with the bulk g-C3N4, which was in agreement with the report,35 suggesting the band gap increase of the gC3N4 nanosheets. Furthermore, a strong absorption of the gC3N4 nanosheets in the near UV region (∼290 nm) made it possible for absorbing laser energy and transferring energy to analyte. C


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Figure 3. Mass spectra of four nucleobases by using (A) CHCA matrix in positive ion mode; (B) 3-AQ matrix in negative ion mode; (C) Graphene matrix in positive ion mode; (D) Graphene matrix in negative ion mode; (E) g-C3N4 nanosheet matrix in positive ion mode; and (F) g-C3N4 nanosheet matrix in negative ion mode. The concentration of each analyte was set as 1 mM. Laser intensity: 60%.

the S/N ratio of 3.7, even when 1 μM His was applied. The detection limit of His obtained on g-C3N4 matrix was much lower than that of graphene matrix (Figure S9, Supporting Information). The significant enhancement in detection sensitivity may be attributed to high LDI efficiency of the gC3N4 nanosheets. The reproducibility was also evaluated by analyzing 0.03 mM His and the results are shown in Figures S10 and S11 (Supporting Information). The signal intensity of 15 acquisition times for one sample spot was stabilized at around 230 and the relative standard deviation (RSD) was calculated to be 10.3% (n = 15). Even at high salt concentration (500 mM NaCl), the RSD of His signal variations was less than 19.3% (n = 15), validating good shot-to-shot reproducibility. Additionally, good spot-to-spot reproducibility was also obtained with RSD of 13.3% (n = 10). MALDI-TOF MS of Nucleobases. Four nucleobases (uracil (U), thymine (T), adenine (A), and guanine (G)) were further examined. Using the CHCA matrix, the multiple nucleobasebased positive ions were detected; taking A as an example, the m/z at 136.13, 158.02, 174.04, 180.03, 196.05, and 212.05 were accordingly assigned to [A + H]+, [A + Na]+, [A + K]+, [A + 2Na−H]+, [A + Na + K−H]+, and [A + 2K−H]+ ions (Figure 3A). Among them, many analyte peaks (e.g., [A + K]+ and [G + Na]+; [A + Na + K − H]+ and [G + 2Na − H]+; [A + 2K − H]+ and [G + Na + K − H]+) overlapped each other, which made it difficult to identify. Although the deprotonated ions of the four nucleobases were well detected with 3-AQ matrix in negative ion mode, many matrix-related ions interfered with the analytes (Figure 3B). Presented in Figure 3C,E were the positive ion analysis of four nucleobases by using graphene and g-C3N4 nanosheets as matrices, where the [M + Na]+, and [M + 2Na − H]+ ions of the four nucleobases predominated the spectrum. In contrast with positive ion mode, negative ion mode of graphene and g-C3N4 nanosheet matrix gave a relatively clear background and thus the exclusive [M-H]− ions of the four nucleobases were unambiguously detected (Figure 3D,F). However, the g-C3N4 nanosheet matrix exhibited an

was detected with CHCA matrix (data not shown), indicating the infeasibility of CHCA matrix in assisting negative ionization. 3-AQ, commonly used in negative ion mode, was also investigated and the result (Figure 2B) showed that the deprotonated [M − H]− ion of His was obtained. However, the signal response of His was rather low. By using graphene matrix, the multiple His-related peaks were identified (Figure 2C). Figure 2D showed the negative ionization of His with graphene matrix. It was seen that although the signal intensity of His produced on graphene matrix was much higher than that of 3-AQ matrix, the presence of background peaks complicated spectrum interpretation. Positive ionization of His on g-C3N4 nanosheet matrix exhibited similar ion peaks as like as graphene (Figure 2E). On the contrary, negative ionization provides a relatively clear background for the analysis of His. As shown in Figure 2F, the only characteristic [M − H]− ion at m/z 154.02 was observed with g-C3N4 nanosheet matrix. Moreover, the signal-to-noise (S/N) ratio of His obtained with g-C3N4 nanosheet matrix was much higher than that those of organic matrices and graphene matrix. In addition, the bulk g-C3N4 was also used as matrix to assist the ionization of His and the result (Figure S6, Supporting Information) showed that no deprotonated ion of His was detected in negative ion mode, suggesting that the bulk g-C3N4 was not as effective as the gC3N4 nanosheet matrix. Similarly, negative ionization of five amino acid (Asp (MW = 133.11), Gln (MW = 146.14), His (MW = 155.15), Phe (MW = 165.19), and Arg (MW = 174.19)) was performed with graphene and g-C3N4 nanosheet matrices. The deprotonated ions of the analytes were all detected as presented in Figure S7 (Supporting Information). Moreover, a clear background with distinct signal enhancement was also observed by using g-C3N4 nanosheet matrix, demonstrating its outstanding performance. The detection sensitivity was further evaluated by analyzing His. As shown in Figure S8 (Supporting Information), the signal intensities and S/N ratio of His were gradually reduced with the decrease of His concentration while using the g-C3N4 matrix. The [M − H]− ion of His could be clearly detected with D


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Figure 4. Mass spectra of five peptides by (A) CHCA matrix in positive ion mode; (B) 3-AQ matrix in negative ion mode; (C) Graphene matrix in positive ion mode; (D) Graphene matrix in negative ion mode; (E) g-C3N4 nanosheet matrix in positive ion mode; and (F) g-C3N4 nanosheet matrix in negative ion mode. The concentrations of Ala-Gln, Gly-Phe, Tyr-Phe, Glu-Val-Phe, and Phe-Gly-Phe-Gly were adjusted to 1.0, 1.0, 0.5, 0.5, and 0.5 mM, respectively. Laser intensity: 70%.

some unknown ion peaks in the mass spectra (Figure S12, Supporting Information). As increasing NaCl concentration up to 500 mM, the signals of the peptides only dropped by less than 5%, indicating the good salt tolerance of the g-C3N4 nanosheets as matrix. MALDI-TOF MS of Bisphenols. Bisphenols (BPs), including bisphenol A (BPA, MW = 228.29), bisphenol B (BPB, MW = 242.31), bisphenol F (BPF, MW = 200.23), and bisphenol S (BPS, MW = 250.27), were known as endocrinedisrupting chemicals and its adverse effects on immune systems in humans have been demonstrated.36 Development of MALDI MS-based approach for direct BPs determination is highly required because of its simple and easy operation. However, we also know that none of the selected standard BPs (except BPS) can be detected with CHCA or 3-AQ matrices in positive- and negative-ion modes. Herein, the g-C3N4 nanosheets used as matrix for BPs analysis was performed to evaluate the performance. As presented in Figure 5A−D, the four BPs were clearly detected with the assistance of the g-C3N4 nanosheets in negative ion mode, where the MS peaks were accordingly assigned to the deprotonated ion [M − H]− (m/z 227.10) and its fragment ion [M − CH4−H]− (m/z 211.16) for BPA, the [M − H]− (m/z 241.03), [M − CH4−H]− (m/z 225.16), and [M − C2H6−H]− (m/z 211.15) ions for BPB, the [M − H]− (m/z 199.10) and its hydrogen rearrangement ions (m/z 197.10) for BPF, and the [M − H]− (m/z 249.00) ion for BPS, respectively (the fragmentation pathways of BPs were showed in Figure S13, Supporting Information). It should be noted that the signal intensities and S/N ratios of the four BPs produced by the g-C3N4 nanosheets were much higher than those obtained with graphene, and thus the relatively low detection limits of the BPs in range of 1.0−3.0 pmol can be

increased enhanced S/N ratios compared with graphene, revealing the superiority of g-C3N4 nanosheets as matrix. MALDI-TOF MS of Peptides. Encouraged by the above results, five short-chain peptides containing Ala-Gln (MW = 217.22), Gly-Phe (MW = 222.24), Tyr-Phe (MW = 328.14), Glu-Val-Phe (MW = 393.19), and Phe-Gly-Phe-Gly (MW = 426.19) were also examined. Figure 4A showed the positive-ion mass spectra of the five peptides with CHCA matrix. Taking Gly-Phe as an example, the peptide-related ions including [M + H]+ (m/z 223.00), [M + Na]+ (m/z 245.02), [M + K]+ (m/z 261.20), and [M + Na + K − H]+ (m/z 283.21) were found. These multiple peptide-related positive ions complicated the mass spectrum. By using 3-AQ matrix, however, the five peptides were well detected in negative ion mode. Although high MS signal responses of the peptides were obtained, the matrix-related peaks appeared accordingly, seriously interfering with the signals of the peptides (Figure 4B). By using graphene in positive ion mode, all of peptides were observed, which was assigned to [M + Na]+, [M + 2Na − H]+, and [M + Na + K − H]+ ions (Figure 4C). Similarly, multiple alkali metal ion complexes of peptides were also detected with g-C 3N4 nanosheet matrix in positive ion mode, where the ion peaks were in the form of [M + Na]+, [M + K]+, and [M + Na + K − H]+ ions (Figure 4E and Table S1, Supporting Information). As expected, however, the exclusive [M − H]− ions of the five peptides were obtained at the aid of graphene and g-C3N4 nanosheet matrices (Figure 4D,F). As compared to graphene, clean background and sensitive characteristic ions were achieved in g-C3N4 nanosheet, displaying its good performance. In the assessment of salt tolerance, adding different concentrations of NaCl from 0 to 500 mM has slight effect on the signal intensities of the five peptides despite having E


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Figure 5. Mass spectra of BPs by using g-C3N4 nanosheets and graphene matrices in negative ion mode: (A) BPA; (B) BPB; (C) BPF; and (D) BPS. The concentration of each analyte was set as 1 mM. Laser intensity: 60%.

Figure 6. Mass spectra of nitro-PAHs by using g-C3N4 nanosheets and graphene matrices in negative ion mode: (A) 1-NP; (B) 6-NC; (C) 9-NA; and (D) 2-NF. The concentration of each analyte was set as 1 mM. Laser intensity: 60%.

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substrate of real sample has slightly influence on the target analysis. Ionization Mechanism of g-C3N4 Nanosheets as Matrix. The good performance of the g-C3N4 nanosheet matrix leads us to examine the effect of the g-C3N4 nanosheet structure and composition on ionization efficiency. It is wellknown that π-conjugated network plays an important role in assisting D/I process, because laser absorption and energy transfer is strongly dependent on the continuous π-conjugated structure.16,37 As displayed in Figure S16A (Supporting Information), three fitted peaks at 284.5, 285.6, and 288.1 eV were observed in the high resolution C 1s XPS spectrum, in which the major C peaks at 288.1 and 284.5 eV were identified as sp2 bonded carbon (N−CN) and graphitic carbon, validating the existence of the continuous π-conjugated structure. Meanwhile, three peaks at 398.48, 399.43, and 400.99 eV were observed from N 1s XPS spectrum (Figure S16B, Supporting Information). The result indicated the existence of pyridinic, pyrrolic, and graphitic nitrogens in the g-C3N4 nanosheets. Since pyridinic nitrogen atoms possessed one pair of sp2 electrons in the conjugated ring system, it can act as a Lewis base38 and tend to accept protons from the analytes. As a result, pyridinic nitrogen in g-C3N4 nanosheets was supposed to speed up the charging process in the negative ion mode and promote an increase in negative ions.26 It may be the reason why the signal intensities of small molecules obtained on g-C3N4 nanosheet matrix are higher than those produced by graphene.

achieved accordingly. The results validated again the benefit of the g-C3N4 nanosheets as matrix for negative ion SALDI MS. MALDI-TOF MS of Nitro-polycyclic Aromatic Hydrocarbons. Nitro-PAHs, as a class of genotoxic environmental pollutants, are often found in air and aquatic systems. Due to the potential adverse effects on human health, development of a sample analytical method for nitro-PAHs is highly desirable. Figure 6A-D presented the analysis of 1-nitropyrene (1-NP), 6nitrochrysene (6-NC), 9-nitroanthracene (9-NA), and 2nitrofluorene (2-NF), where the [M + O−H]− and [M − NO]− ions for 1-NP and 6-NC, the [M − NO]− ion for 9-NA, and the [M − NO]−, [M − H]−, [M + O−H]−, and [M + O− H + NO−H]− for 2-NF were clearly observed with the assistance of the g-C3N4 nanosheets and graphene. However, a 1−3-fold signal enhancement of nitro-PAHs was obtained using the g-C3N4 nanosheet matrix compared with graphene, and the corresponding detection limit of the nitro-PAHs reached 1.0− 2.5 pmol. The above results encourage us to further explore its potential application in real sample. Determination of 1-NP in Sewage Sample. Figure S14 (Supporting Information) displayed the quantitative analysis of 1-NP standard with different concentrations. It was observed that the S/N ratios of 1-NP standard decreased from 528.5 to 6.8 with the decrease of 1-NP concentration from 40 to 1 μM. Furthermore, a linear relationship was obtained between the ion intensity and its concentration of 1-NP from 1 to 40 μM. Correspondingly, the regression equation was y = 69.1268x + 0.00015 (R2 = 0.9965; Figure S15, Supporting Information). To further illustrate the applicability, determination of 1-NP in sewage was evaluated. Figure 7 presented the MS peaks of the

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CONCLUSIONS In summary, g-C3N4 nanosheets were prepared by facile liquid exfoliation and utilized as a new matrix for negative ion MALDI-TOF MS. A wide range of small molecules including amino acids, peptides, nucleobases, BPs, and nitro-PAHs were investigated, and the results demonstrated the great advantages of the g-C3N4 nanosheet matrix over organic matrices (e.g., CHCA, 3-AQ) and graphene matrix, including low background interference, high sensitivity, good salt tolerance, excellent stability, and reproducibility. In addition, the quantitative analysis of 1-NP in real sample suggested the potential application of this matrix. It is expected that the g-C3N4 nanosheet-assisted LDI MS method can be extended to other small molecule analysis.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional data as noted in text. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02066.

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Figure 7. Determination of 1-NP in sewage by g-C3N4 nanosheetassisted LDI MS in negative ion mode. Laser intensity: 60%.

AUTHOR INFORMATION

Corresponding Author

*Fax: +852-34117348. E-mail: [email protected]. Notes

sewage sample with and without addition of 1-NP. There are several unknown ion peaks appeared in the range of m/z 120− 175 and no signal of 1-NP was detected in the blank sewage. However, an unambiguous [M-NO]− ion of 1-NP at m/z 217.00 was obtained with the S/N ratio of 6.7 after spiking 1 μM 1-NP (equivalent to 1 pmol). It should be noted that the signal intensity of 1-NP in sewage can stand comparison with that in standard test solution, suggesting that the complex

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from National Natural Science Foundation of China (21375018 and 21275020), and the Natural Science Foundation of Fujian Province (2014J01402). G


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