Supporting Information

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1 Supporting Information Facile Synthesis of N-Doped Carbon Dots as A New Matrix for Detection of Hydroxy-Polycyclic Aromatic Hydrocarbons by Negative-Ion Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry Wenjing Lu,, Yong Li, Ruijin Li, Shaomin Shuang, Chuan Dong, *, *,, Zongwei Cai Institute of Environmental Science, and School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan , 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 Exchange student on visit to Hong Kong Baptist University. Corresponding author Chuan Dong, Fax: ; dc@sxu.edu.cn Zongwei Cai, Fax: ; zwcai@hkbu.edu.hk S-1

2 EXPERIMENTAL SECTION Chemicals and Materials. DL-malic acid and ethanolamine were purchased from Aladdin (L.A., USA). The hydroxy-pahs standards including 1-hydroxy-pyrene (1-OH-Pyr), 1, 4-dihydroxy-anthraquinone (1, 4-OH-AntQn) and 11, 12-dihydroxy-benzo(a)pyrene (11, 12-OH-BaP) were purchased from J&K Chemical Ltd. α-cyano-4-hydrox-ycinnamicacid (CHCA) 2, 5-Dihydroxybenzoic acid (DHB) and trifluoroacetic acid (TFA) were obtained from Alfa Aesar (Ward Hill, MA, USA). Acetonitrile (ACN), dichloromethane (DCM) and all other reagents were analytical grade. Ultrapure deionized water ( MΩ cm) from a Milli-Q Plus system (Millipore, Bedford, MA, USA) was used in all experiments. PM 2.5 samples were collected over 24-hour period with quartz fiber filters (QFFs, 22mm in diameter) from the roof of Institute of Environmental Science of Shanxi University using an active sampler (Thermo Anderson, USA) at a flow rate of 100 L /min about 25 meters high from the ground. Concurrent sampling of PM 2.5 aerosols on a rooftop of a 5-story building (about 25 m above ground) was conducted at a location in Taiyuan, China (Shanxi University, 30 15'N, 'E) during the ash haze weather from December, This site is located approximately 300 m away from a major roadway (Wucheng road). There are no obvious industrial pollution sources around. PM 2.5 concentrations were measured using DustTrak II Aerosol Monitor (TSI Inc., USA). S-2

3 Synthesis of Undoped-CDs and N-CDs. The CDs were synthesized by a facile green route of microwave assisted pyrolysis method. For undoped-cds, g (4 mm) DL-Malic acid was dissolved in 10 ml ultrapure water under ultrasonic stirring for 30 min to form a homogeneous solution. Then, the transparent solution was put into a domestic microwave oven and heated for 10 min. The product was dissolved in 20 ml of ultrapure water after cooled down to room temperature. Then the solution was filtered and dialyzed against ultrapure water through a dialysis membrane (MWCO ¼ ) for 72 hours. Finally, a light yellow aqueous solution was lyophilized to collect dry undoped-cds. For N-CDs, g (2 mm) DL-Malic acid was dissolved in 10 ml ultrapure water, and then 242 µl (4mM) Ethanolamine was added to the solution. The obtained solution was treated in the same way as those for undoped-cds. Characterization. A JEM-2100 electron microscope (Tokyo, Japan) operating at 300 kv was employed to obtain the transmission electron microscopic (TEM) and high resolution transmission electron microscopy (HRTEM) images. The Fourier transform infrared (FT-IR) spectra were recorded on a Bruker TensorⅡ FT-IR spectrometer (Bremen, Germany). The crystal structure of three CDs were characterized by a Bruker D8 Advance X-ray diffractometer (λ= nm). X-ray Photoelectron Spectroscopy (XPS) was recorded by a Shimadzu/Kratos AXIS ULTRA DLD photoelectron spectrometer (Kratos, Tokyo, Japan). The UV/VIS absorption spectra were acquired on a Lambda 365 UV/VIS Spectrophotometer (PerkinElmer Co., Ltd., America). The PL spectra were carried out on a Varian Cary Eclipse S-3

4 fluorescence spectrophotometer. Preparation of Analyte Solution. Stock solutions of OH-PAHs standards at concentration of 1.0 µg/µl were prepared by dissolving proper amount OH-PAHs in ACN/DCM (1:1 v/v). All of the above solutions and suspension were stored at 4 C. PM 2.5 quartz fiber membrane was divided into 1 cm 1 cm size, and then extracted with two 25 ml ACN/DCM (1:1 v/v) under ultrasonic assistance in 50 ml conical flask. The extracts were dried under a stream of nitrogen. Then the residue was scattered into 100 µl ACN/DCM (1:1 v/v). The sample was analyzed by MALDI-TOF MS. MALDI-TOF MS Analysis. 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 ~60% in positive and negative reflection mode. Acquisition parameters included 19 kv acceleration voltages and delayed extraction at 120 ns. Each recorded mass spectrum was generated by averaging data from 500 individual laser shots. In order to avoid fragmentation of the analytes, the laser power was adjusted to slightly above the D/I threshold energy. Mass calibrations were performed externally using the mass peaks of CHCA and 2, 5-dihydroxybenzoic acid (DHB) for small molecule analysis. CHCA N-CDs S-4

5 Fig. S1. The optical images of CHCA, and N-CDs with the analyte solutions dispersed on the stainless steel target. Matrix concentration: CHCA (10 mg/ml) and N-CDs (1.0 mg/ml). Fig. S2. MALDI mass spectra of N-CDs in positive ion mode (A) and negative ion mode (B). Laser intensity: 60%. Fig. S3. Fragmentation pathways of 11, 12-OH-BaP. Table S1 Analytical parameters of the proposed MALDI-TOF MS method S-5

6 Analytes Linear range (ng/µl) Linear equation (ng/µl) R 2 LOD (ng/µl) 1-OH-Pyr y= x , 4-OH-AntQn y= x , 12-OH-BaP y= x The detection limits were calculated by using S/N=3. Fig. S4. MS signal intensity of 1-OH-pyr repeatedly acquired from one sample spot by using N-CDs matrix in negative ion mode. Continuous 15 spectra were obtained by applying laser shots on random positions uniformly located on the spot. Laser intensity: 60%. Fig. S5. MS signal intensity of 1-OH-pyr repeatedly acquired from 10 different sample spots by using N-CDs matrix in negative ion mode. Laser intensity: 60%. Table S2. The detection limits of OH-Pyr from different analytical methods. S-6

7 Quantified Detection method LOD ng/m 3 The source of sample Reference compound name 1-OH-Pyr GC-EI/MS personal PM 2.5 sample 44 1-OH-Pyr GC-ECNI/MS PM 2.5 sample in Beijing in heating 68 seasons Σ-OH-Pyr MALDI-TOFMS PM 2.5 sample in Taiyuan in heating season This work* S-7