Academic Appointments

All Publications

  • The power of microdroplet photochemistry. Chemical science Song, X., Zare, R. N. 2024; 15 (10): 3670-3672


    This study presents compelling evidence demonstrating that irradiation of the air-solution interface, whether achieved through the spraying of microdroplets into the air or by bubbling air through a solution, significantly accelerates the rate of photochemical reactions by orders of magnitude compared to identical reaction conditions in bulk solutions. We propose this approach as a novel and versatile method for harnessing solar energy in chemical transformations.

    View details for DOI 10.1039/d4sc00056k

    View details for PubMedID 38454998

    View details for PubMedCentralID PMC10915808

  • The power of microdroplet photochemistry CHEMICAL SCIENCE Song, X., Zare, R. N. 2024

    View details for DOI 10.1039/d4sc00056k

    View details for Web of Science ID 001156639100001

  • Continuous ammonia synthesis from water and nitrogen via contact electrification. Proceedings of the National Academy of Sciences of the United States of America Li, J., Xia, Y., Song, X., Chen, B., Zare, R. N. 2024; 121 (4): e2318408121


    We synthesized ammonia (NH3) by bubbling nitrogen (N2) gas into bulk liquid water (200 mL) containing 50 mg polytetrafluoroethylene (PTFE) particles (~5 µm in diameter) suspended with the help of a surfactant (Tween 20, ~0.05 vol.%) at room temperature (25 °C). Electron spin resonance spectroscopy and density functional theory calculations reveal that water acts as the proton donor for the reduction of N2. Moreover, isotopic labeling of the N2 gas shows that it is the source of nitrogen in the ammonia. We propose a mechanism for ammonia generation based on the activation of N2 caused by electron transfer and reduction processes driven by contact electrification. We optimized the pH of the PTFE suspension at 6.5 to 7.0 and employed ultrasonic mixing. We found an ammonia production rate of ~420 μmol L-1 h-1 per gram of PTFE particles for the conditions described above. This rate did not change more than 10% over an 8-h period of sustained reaction.

    View details for DOI 10.1073/pnas.2318408121

    View details for PubMedID 38232282

  • Noninvasive Detection of Skin Cancer by Imprint Desorption Electrospray Ionization Mass Spectrometry Imaging. Analytical chemistry Meng, Y., Chiou, A. S., Aasi, S. Z., See, N. A., Song, X., Zare, R. N. 2023


    We report a technique for the noninvasive detection of skin cancer by imprint desorption electrospray ionization mass spectrometry imaging (DESI-MSI) using a transfer agent that is pressed against the tissue of interest. By noninvasively pressing a tape strip against human skin, metabolites, fatty acids, and lipids on the skin surface are transferred to the tape with little spatial distortion. Running DESI-MSI on the tape strip provides chemical images of the molecules on the skin surface, which are valuable for distinguishing cancer from healthy skin. Chemical components of the tissue imprint on the tape strip and the original basal cell carcinoma (BCC) section from the mass spectra show high consistency. By comparing MS images (about 150-μm resolution) of same molecules from the tape strip and from the BCC section, we confirm that chemical patterns are successfully transferred to the tape stripe. We also used the technique to distinguish cherry angiomas from normal human skin by comparing the molecular patterns from a tape strip. These results demonstrate the potential of the imprint DESI-MSI technique for the noninvasive detection of skin cancers as well as other skin diseases before and during clinical surgery.

    View details for DOI 10.1021/acs.analchem.3c04918

    View details for PubMedID 38155587

  • Water Microdroplets-Initiated Methane Oxidation. Journal of the American Chemical Society Song, X., Basheer, C., Zare, R. N. 2023


    The special redox reactivity of water microdroplets causes "mild ignition" of methane gas to form methane oxygenates. The C(sp3)-H bond of methane can be activated by the hydroxyl radical (OH·) or the hydrogen radical (H·) across the air-water interface (AWI) of microdroplets to generate the methyl radical (CH3·). Once CH3· is formed, it undergoes free-radical reactions with O2 in the air, excessive OH· and H· across the AWI, and H2O2 present at the AWI and generated CH3· itself to produce methanol and other species. Production of the methanol and other oxygenates was confirmed by gas chromatography, mass spectrometry, and 1H- and 13C-nuclear magnetic resonance. Formic acid, acetic acid, ethanol, carbon dioxide, and methyl peroxide were also detected as methane oxidation byproducts. This water microdroplet-initiated oxidation process can be further enhanced under ultrasonication to yield 2.66 ± 0.77 mM methanol conversion from the methane gas in a single spray run for 30 min, with a selectivity of 19.2% compared with all other oxygenated species.

    View details for DOI 10.1021/jacs.3c08643

    View details for PubMedID 38054976

  • One-step Formation of Urea from Carbon Dioxide and Nitrogen Using Water Microdroplets. Journal of the American Chemical Society Song, X., Basheer, C., Xia, Y., Li, J., Abdulazeez, I., Al-Saadi, A. A., Mofidfar, M., Suliman, M. A., Zare, R. N. 2023


    Water (H2O) microdroplets are sprayed onto a graphite mesh covered with a CuBi2O4 coating using a 1:1 mixture of N2 and CO2 as the nebulizing gas. The resulting microdroplets contain urea [CO(NH2)2] as detected by both mass spectrometry and 13C nuclear magnetic resonance. This gas-liquid-solid heterogeneous catalytic system synthesizes urea in one step on the 0.1 ms time scale. The conversion rate reaches 2.7 mmol g-1 h-1 at 25 °C and 12.3 mmol g-1 h-1 at 65 °C, with no external voltage applied. Water microdroplets serve as the hydrogen source and the electron transfer medium for N2 and CO2 in contact with CuBi2O4. Water-gas and water-solid contact electrification are speculated to drive the reaction process. This strategy couples N2 fixation and CO2 utilization in an ecofriendly process to produce urea, converting a greenhouse gas into a value-added product.

    View details for DOI 10.1021/jacs.3c10784

    View details for PubMedID 37966066

  • Making ammonia from nitrogen and water microdroplets. Proceedings of the National Academy of Sciences of the United States of America Song, X., Basheer, C., Zare, R. N. 2023; 120 (16): e2301206120


    Water (H2O) microdroplets are sprayed onto a magnetic iron oxide (Fe3O4) and Nafion-coated graphite mesh using compressed N2 or air as the nebulizing gas. The resulting splash of microdroplets enters a mass spectrometer and is found to contain ammonia (NH3). This gas-liquid-solid heterogeneous catalytic system synthesizes ammonia in 0.2 ms. The conversion rate reaches 32.9 ± 1.38 nmol s-1 cm-2 at room temperature without application of an external electric potential and without irradiation. Water microdroplets are the hydrogen source for N2 in contact with Fe3O4. Hydrazine (H2NNH2) is also observed as a by-product and is suspected to be an intermediate in the formation of ammonia. This one-step nitrogen-fixation strategy to produce ammonia is eco-friendly and low cost, which converts widely available starting materials into a value-added product.

    View details for DOI 10.1073/pnas.2301206120

    View details for PubMedID 37036968

  • Silicone Wristband Spray Ionization Mass Spectrometry for Combined Exposome and Metabolome Profiling ISRAEL JOURNAL OF CHEMISTRY Mofidfar, M., Song, X., Kelly, J. T., Rubenstein, M. H., Zare, R. N. 2023
  • Immuno-Desorption Electrospray Ionization Mass Spectrometry Imaging Identifies Functional Macromolecules by Using Microdroplet-Cleavable Mass Tags. Angewandte Chemie (International ed. in English) Song, X., Zang, Q., Li, C., Zhou, T., Zare, R. N. 2023


    We present immunoassay-based desorption electrospray ionization mass spectrometry imaging (immuno-DESI-MSI) to visualize functional macromolecules such as drug targets and cascade signaling factors. A set of boronic acid mass tags (BMTs) were synthesized to label antibodies as MSI probes. The boronic ester bond is employed to cross-link the BMT with the galactosamine-modified antibody. The BMT can be released from its tethered antibody by ultrafast cleavage of the boronic ester bond caused by the acidic condition of sprayed DESI microdroplets containing water. The fluorescent moiety enables the BMT to work in both optical and MS imaging modes. The positively charged quaternary ammonium group enhances the ionization efficiency. The introduction of the boron element also makes mass tags readily identified because of its unique isotope pattern. Immuno-DESI-MSI provides an appealing strategy to spatially map macromolecules beyond what can be observed by conventional DESI-MSI, provided antibodies are available to the targetted molecules of interest.

    View details for DOI 10.1002/anie.202216969

    View details for PubMedID 36622964

  • Reply to Brzeski and Jordan: Potential pyridine tautomers that can form stable dipole-bound anions. Proceedings of the National Academy of Sciences of the United States of America Zhao, L., Song, X., Gong, C., Zhang, D., Wang, R., Zare, R. N., Zhang, X. 2022; 119 (38): e2212433119

    View details for DOI 10.1073/pnas.2212433119

    View details for PubMedID 36095181

  • Spraying Water Microdroplets Containing 1,2,3-Triazole Converts Carbon Dioxide into Formic Acid. Journal of the American Chemical Society Song, X., Meng, Y., Zare, R. N. 2022


    We report the use of 1,2,3-triazole (Tz)-containing water microdroplets for gas-phase carbon dioxide (CO2) reduction at room temperature. Using a coaxial sonic spraying setup, the CO2 can be efficiently captured by Tz and converted to formic acid (HCOOH; FA) at the gas-liquid interface (GLI). A mass spectrometer operated in negative ion mode monitors the capture of CO2 to form the bicarbonate anion (HCO3-) and conversion to form the formate anion (HCOO-). Varied FA species were successfully identified by MS/MS experiments including the formate monomer ([FA - H]-, m/z 45), the dimer ([2FA - H]-, m/z 91; [2FA + Na - 2H]-, m/z 113), the trimer ([3FA - H]-, m/z 137), and some other adducts (such as [FA - H + H2CO3]-, m/z 107; [2FA + Na - 2H + Tz]-, m/z 182). The reaction conditions were systematically optimized to make the maximum conversion yield reach over 80% with an FA concentration of approximately 71 ± 3.1 μM. The mechanism for the reaction is speculated to be that Tz donates the proton and the hydroxide (OH-) at the GLI, resulting in a stepwise yield of electrons to reduce gas-phase CO2 to FA.

    View details for DOI 10.1021/jacs.2c07779

    View details for PubMedID 36075012

  • Laser Ablation Electrospray Ionization Achieves 5 mum Resolution Using a Microlensed Fiber. Analytical chemistry Meng, Y., Song, X., Zare, R. N. 2022


    A pulsed (10 Hz) infrared (IR) (1064 nm) laser is focused on a sample surface by means of a microlensed fiber. Analytes desorbed from the surface are captured by charged microdroplets before entering a mass spectrometer. By translating the sample surface, a chemical map is generated with a resolution of 5 mum, defined as the change from 20 to 80% of the analyte signal intensity. As a demonstration of the power of this new imaging technique, analytes from a parsnip root section are imaged and compared with that obtained from conventional laser ablation electrospray ionization mass spectrometry. The improvement in spatial resolution is about a factor of 20.

    View details for DOI 10.1021/acs.analchem.2c01942

    View details for PubMedID 35797218

  • Capture of Hydroxyl Radicals by Hydronium Cations in Water Microdroplets. Angewandte Chemie (International ed. in English) Xing, D., Meng, Y., Yuan, X., Jin, S., Song, X., Zare, R. N., Zhang, X. 2022


    Despite the high stability of bulk water, water microdroplets possess strikingly different properties, such as the presence of hydroxyl radicals (OH) at the air-water interface. Previous studies exhibited the recombination of OH into H 2 O 2 molecules and the capture of OH by oxidizing other molecules. By spraying pure water microdroplets into a mass spectrometer, we detected OH in the form of (H 4 O 2 ) + that is essentially OH--H 3 O + , a hydroxyl radical combined with a hydronium cation through hydrogen bonding. We also successfully captured it with two OH scavengers, caffeine and melatonin, and key oxidation radical intermediates that bear important mechanistic information were seen. It is suggested that some previous reactions involving (H 4 O 2 ) + should be attributed to reactions with OH--H 3 O + rather than with the water dimer cation (H 2 O-OH 2 ) + .

    View details for DOI 10.1002/anie.202207587

    View details for PubMedID 35700155

  • Sprayed water microdroplets containing dissolved pyridine spontaneously generate pyridyl anions. Proceedings of the National Academy of Sciences of the United States of America Zhao, L., Song, X., Gong, C., Zhang, D., Wang, R., Zare, R. N., Zhang, X. 2022; 119 (12): e2200991119


    SignificanceWater microdroplets can accelerate chemical reactions by orders of magnitude compared to the same reactions in bulk water and/or trigger spontaneous reactions that do not occur in bulk solution. Among the properties of water microdroplets, the unique redox ability resulting from the spontaneous dissociation of OH- into a released electron and OH at the air-water interfaces is especially intriguing. At the air-water interface, OH- exhibits a strong reducing potential, and the resulting OH is highly oxidative, making water microdroplets a unity of opposites. We report the reduction of pyridine into pyridyl anions (C5H5N-) and the oxidation of pyridine into hydroxypyridine, which extends what we know about the redox power of water microdroplets.

    View details for DOI 10.1073/pnas.2200991119

    View details for PubMedID 35286201

  • Hydrogen-Deuterium Exchange Desorption Electrospray Ionization Mass Spectrometry Visualizes an Acidic Tumor Microenvironment. Analytical chemistry Song, X., Zang, Q., Zare, R. N. 2021


    We report that microdroplet hydrogen-deuterium exchange (HDX) detected by desorption electrospray ionization mass spectrometry imaging (DESI-MSI) allows the measurement of the acidity of a tissue sample. The integration of HDX and DESI-MSI has been applied to visualize the acidic tumor microenvironment (TME). HDX-DESI-MSI enables the simultaneous collection of regional pH variation and its corresponding in-depth metabolomic changes. This technique is a cost-effective tool for providing insight into the pH-dependent tumor metabolism heterogeneity.

    View details for DOI 10.1021/acs.analchem.1c02026

    View details for PubMedID 34279072

  • Distinguishing between Isobaric Ions Using Microdroplet Hydrogen–Deuterium Exchange Mass Spectrometry Metabolites Song, X., Li, J., Mofidfar, M., Zare, R. N. 2021

    View details for DOI 10.3390/metabo11110728

  • In situ DESI-MSI lipidomic profiles of mucosal margin of oral squamous cell carcinoma. EBioMedicine Yang, X., Song, X., Zhang, X., Shankar, V., Wang, S., Yang, Y., Chen, S., Zhang, L., Ni, Y., Zare, R. N., Hu, Q. 2021; 70: 103529


    Although there is consensus that the optimal safe margin is ≥ 5mm, obtaining clear margins (≥5 mm) intraoperatively seems to be the major challenge. We applied a molecular diagnostic method at the lipidomic level to determine the safe surgical resection margin of OSCC by desorption electrospray ionisation mass spectrometry imaging (DESI-MSI).By overlaying mass spectrometry images with hematoxylin-eosin staining (H&E) from 18 recruited OSCC participants, the mass spectra of all pixels across the diagnosed tumour and continuous mucosal margin regions were extracted to serve as the training and validation datasets. A Lasso regression model was used to evaluate the test performance.By leave-one-out validation, the Lasso model achieved 88.6% accuracy in distinguishing between tumour and normal regions. To determine the safe surgical resection distance and margin status of OSCC, a set of 14 lipid ions that gradually decreased from tumour to normal tissue was assigned higher weight coefficients in the Lasso model. The safe surgical resection distance of OSCC was measured using the developed 14 lipid ion molecular diagnostic model for clinical reference. The overall accuracy of predicting tumours, positive margins, and negative margins was 92.6%.The spatial segmentation results based on our diagnostic model not only clearly delineated the tumour and normal tissue, but also distinguished the different status of surgical margins. Meanwhile, the safe surgical resection margin of OSCC on frozen sections can also be accurately measured using the developed diagnostic model.This study was supported by Nanjing Municipal Key Medical Laboratory Constructional Project Funding (since 2016) and the Centre of Nanjing Clinical Medicine Tumour (since 2014).

    View details for DOI 10.1016/j.ebiom.2021.103529

    View details for PubMedID 34391097

  • Big cohort metabolomic profiling of serum for oral squamous cell carcinoma screening and diagnosis Natural Sciences Yang, X., Song, X., Yang, X., Han, W., Fu, Y., Wang, S., Zhong, X., Lu, Y., Sun, G., Wang, Z., Ni, Y., Zare, R. N., Hu, Q. 2021; 1 (1)
  • Introducing Nafion for In Situ Desalting and Biofluid Profiling in Spray Mass Spectrometry. Frontiers in chemistry Song, X., Mofidfar, M., Zare, R. N. 2021; 9: 807244


    We introduce Nafion into the ambient ionization technique of spray mass spectrometry to serve for in situ desalting and direct analysis of biological fluids. Nafion was coated onto the surface of the triangular spray tip as the cation exchange material. Because the sulfonic group from the Nafion membrane effectively exchanges their carried protons with inorganic salt ions (e.g., Na+ and K+), the analyte's ionization efficiency can be significantly enhanced by reducing ion suppression. The desalting efficiency can reach 90% and the maximum tolerance of the absolute salt amount reaches 100 μmol. The mass spectral profile can also be simplified by removing the multiple adducted ion types from small-molecule drugs and metabolites ([M + Na]+ and [M + K]+), or multiply charged ions formed by proteins ([M + nNa]n+ and [M + nK]n+). Thus, the Nafion coating makes less ambiguous data interpretation collected from spray mass spectrometry for qualitative profiling or quantitative measurement of a target analyte.

    View details for DOI 10.3389/fchem.2021.807244

    View details for PubMedID 35145954

    View details for PubMedCentralID PMC8821663

  • Coulometry-assisted quantitation in spray ionization mass spectrometry. Journal of mass spectrometry : JMS Song, X., Chen, H., Zare, R. N. 2020: e4628


    The concentration of target analyte in a mixture can be quantified by combining coulometric measurements with spray ionization mass spectrometry. A three-electrode system screen printed on the polymer support acts both as the coulometry platform for electrochemical oxidation and the sample loading tip for spray ionization. After loading a droplet of the analyte solution onto the tip, two steps were taken to implement quantitation. First, the electrochemical oxidation potential was optimized with cyclic voltammetry followed by coulometric measurements to calculate the amount of oxidized analyte under a constant low voltage within a fixed period of time (5 s). Then, a high voltage (+4.5 kV) was applied to the tip to trigger spray ionization for measuring the oxidation yield from the native analyte ion and its oxidized product ion intensities by mass spectrometry. The analyte's native concentration is quantified by dividing the oxidized product's concentration (based on Coulomb's law) and the oxidation yield (estimated from mass spectrometry [MS] assuming that the parent and oxidation product have nearly the same ionization efficiencies). The workflow has an advantage in being free of any standard for constructing the quantitation curve. Several model compounds (tyrosine, dopamine, and angiotensin II) were selected for method validation. It was demonstrated that this strategy was feasible with an accuracy of ~15% for a wide coverage of different species including endogenous metabolites and peptides. As an example of its possible practical use, it was initially employed to make a bilirubin assay in urine.

    View details for DOI 10.1002/jms.4628

    View details for PubMedID 33245185

  • Oral squamous cell carcinoma diagnosed from saliva metabolic profiling. Proceedings of the National Academy of Sciences of the United States of America Song, X., Yang, X., Narayanan, R., Shankar, V., Ethiraj, S., Wang, X., Duan, N., Ni, Y., Hu, Q., Zare, R. N. 2020


    Saliva is a noninvasive biofluid that can contain metabolite signatures of oral squamous cell carcinoma (OSCC). Conductive polymer spray ionization mass spectrometry (CPSI-MS) is employed to record a wide range of metabolite species within a few seconds, making this technique appealing as a point-of-care method for the early detection of OSCC. Saliva samples from 373 volunteers, 124 who are healthy, 124 who have premalignant lesions, and 125 who are OSCC patients, were collected for discovering and validating dysregulated metabolites and determining altered metabolic pathways. Metabolite markers were reconfirmed at the primary tissue level by desorption electrospray ionization MS imaging (DESI-MSI), demonstrating the reliability of diagnoses based on saliva metabolomics. With the aid of machine learning (ML), OSCC and premalignant lesions can be distinguished from the normal physical condition in real time with an accuracy of 86.7%, on a person by person basis. These results suggest that the combination of CPSI-MS and ML is a feasible tool for accurate, automated diagnosis of OSCC in clinical practice.

    View details for DOI 10.1073/pnas.2001395117

    View details for PubMedID 32601197

  • Conductive Polymer Spray Ionization Mass Spectrometry for Biofluid Analysis ANALYTICAL CHEMISTRY Song, X., Chen, H., Zare, R. N. 2018; 90 (21): 12878-12885


    We present a conductive polymer spray ionization (CPSI) method for the direct mass spectrometric analysis of hydrophilic drugs, saccharides, peptides, and proteins in biofluids. Carbon nanotubes (CNTs) were introduced into poly(methyl methacrylate) (PMMA) to fabricate a conductive composite substrate CNT/PMMA in the shape of a triangle (8 mm wide and 10 mm long) with its apex pointed toward the inlet of a mass spectrometer. In comparison with a traditional paper spray substrate, the conductive polymer absorbs less hydrophilic compounds owing to its hydrophobic nature. When aqueous biofluid samples are loaded, they also exhibit less diffusion on this nonporous surface. Only 1.0-2.0 μL solvent suffices to extract the components in a dried biofluid spot and to form charged microdroplets (4.5 kV high voltage applied). Furthermore, the hydrophobic polymer surface only needs to overcome weak surface tension to emit charged microdroplets, so that the signal has a typical duration of 7.5 min. For sunitinib, acarbose, melamine, and angiotensin II, the ion intensity of the target compound from the conductive polymer support is significantly higher than paper spray, typically by a factor of 20 to 100. These results suggest that the CNT/PMMA conductive polymer spray has great potential in the analysis of hydrophilic drugs, saccharides, peptides, and proteins in biofluids.

    View details for DOI 10.1021/acs.analchem.8b03460

    View details for Web of Science ID 000449722500076

    View details for PubMedID 30247892