Academic Appointments


Honors & Awards


  • Young Investigator Award, American Society for Nanomedicine (ASNM) (2009)
  • Best Poster Award, Grodins Graduate Research Symposium (2009)
  • Best Paper Award, Nano Bioelectronics & Systems Research Center (2005)

All Publications


  • Spontaneous formation of gold nanostructures in aqueous microdroplets NATURE COMMUNICATIONS Lee, J., Samanta, D., Nam, H., Zare, R. N. 2018; 9: 1562

    Abstract

    The synthesis of gold nanostructures has received widespread attention owing to many important applications. We report the accelerated synthesis of gold nanoparticles (AuNPs), as well as the reducing-agent-free and template-free synthesis of gold nanoparticles and nanowires in aerosol microdroplets. At first, the AuNP synthesis are carried out by fusing two aqueous microdroplet streams containing chloroauric acid and sodium borohydride. The AuNPs (~7 nm in diameter) are produced within 60 µs at the rate of 0.24 nm µs-1. Compared to bulk solution, microdroplets enhance the size and the growth rate of AuNPs by factors of about 2.1 and 1.2 × 105, respectively. Later, we find that gold nanoparticles and nanowires (~7 nm wide and >2000 nm long) are also formed in microdroplets in the absence of any added reducing agent, template, or externally applied charge. Thus, water microdroplets not only accelerate the synthesis of AuNPs by orders of magnitude, but they also cause spontaneous formation of gold nanostructures.

    View details for DOI 10.1038/s41467-018-04023-z

    View details for Web of Science ID 000430389100014

    View details for PubMedID 29674623

    View details for PubMedCentralID PMC5908806

  • Microdroplet fusion mass spectrometry: accelerated kinetics of acid-induced chlorophyll demetallation QUARTERLY REVIEWS OF BIOPHYSICS Lee, J. K., Nam, H. G., Zare, R. N. 2017; 50: 1-7

    Abstract

    Kinetics of acid-induced chlorophyll demetallation was recorded in microdroplets by fusing a stream of microdroplets containing 40 µM chlorophyll a or b dissolved in methanol with a stream of aqueous microdroplets containing 35 mM hydrochloric acid (pH = 1·46). The kinetics of the demetallation of chlorophyll in the fused microdroplets (14 ± 6 µm diameter; 84 ± 18 m s-1 velocity) was recorded by controlling the traveling distance of the fused microdroplets between the fusion region and the inlet of a mass spectrometer. The rate of acid-induced chlorophyll demetallation was about 960 ± 120 times faster in the charged microdroplets compared with that reported in bulk solution. If no voltage was applied to the sprayed microdroplets, then the acceleration factor was about 580 ± 90, suggesting that the applied voltage is not a major factor determining the acceleration. Chlorophyll a was more rapidly demetallated than chlorophyll b by a factor of ~26 in bulk solution and ~5 in charged microdroplets. The demetallation kinetics was second order in the H+ concentration, but the acceleration factor of microdroplets compared with bulk solution appeared to be unchanged in going from pH = 1·3 to 7·0. The water:methanol ratio of the fused microdroplets was varied from 7:3 to 3:7 causing an increase in the reaction rate of chlorophyll a demetallation by 20%. This observation demonstrates that the solvent composition, which has different evaporation rates, does not significantly affect the acceleration. We believe that a major portion of the acceleration can be attributed to confinement effects involving surface reactions rather than either to evaporation of solvents or to the introduction of charges to the microdroplets.

    View details for DOI 10.1017/S0033583517000014

    View details for Web of Science ID 000396422700001

    View details for PubMedCentralID PMC5729759

  • Abiotic production of sugar phosphates and uridine ribonucleoside in aqueous microdroplets. Proceedings of the National Academy of Sciences of the United States of America Nam, I., Lee, J. K., Nam, H. G., Zare, R. N. 2017; 114 (47): 12396–400

    Abstract

    Phosphorylation is an essential chemical reaction for life. This reaction generates fundamental cell components, including building blocks for RNA and DNA, phospholipids for cell walls, and adenosine triphosphate (ATP) for energy storage. However, phosphorylation reactions are thermodynamically unfavorable in solution. Consequently, a long-standing question in prebiotic chemistry is how abiotic phosphorylation occurs in biological compounds. We find that the phosphorylation of various sugars to form sugar-1-phosphates can proceed spontaneously in aqueous microdroplets containing a simple mixture of sugars and phosphoric acid. The yield for d-ribose-1-phosphate reached over 6% at room temperature, giving a ΔGvalue of -1.1 kcal/mol, much lower than the +5.4 kcal/mol for the reaction in bulk solution. The temperature dependence of the product yield for the phosphorylation in microdroplets revealed a negative enthalpy change (ΔH= -0.9 kcal/mol) and a negligible change of entropy (ΔS= 0.0007 kcal/mol·K). Thus, the spontaneous phosphorylation reaction in microdroplets occurred by overcoming the entropic hurdle of the reaction encountered in bulk solution. Moreover, uridine, a pyrimidine ribonucleoside, is generated in aqueous microdroplets containing d-ribose, phosphoric acid, and uracil, which suggests the possibility that microdroplets could serve as a prebiotic synthetic pathway for ribonucleosides.

    View details for DOI 10.1073/pnas.1714896114

    View details for PubMedID 29078402

    View details for PubMedCentralID PMC5703324

  • Nanotip Ambient Ionization Mass Spectrometry ANALYTICAL CHEMISTRY Zhou, Z., Lee, J. K., Kim, S. C., Zare, R. N. 2016; 88 (10): 5542-5548

    Abstract

    A method called nanotip ambient ionization mass spectrometry (NAIMS) is described, which applies high voltage between a tungsten nanotip and a metal plate to generate a plasma in which ionized analytes on the surface of the metal plate are directed to the inlet and analyzed by a mass spectrometer. The dependence of signal intensity is investigated as a function of the tip-to-plate distance, the tip size, the voltage applied at the tip, and the current. These parameters are separately optimized to achieve sensitivity or high spatial resolution. A partially observable Markov decision process is used to achieve a stabilized plasma as well as high ionization efficiency. As a proof of concept, the NAIMS technique has been applied to phenanthrene and caffeine samples for which the limits of detection were determined to be 0.14 fmol for phenanthrene and 4 amol for caffeine and to a printed caffeine pattern for which a spatial resolution of 8 ± 2 μm, and the best resolution of 5 μm, was demonstrated. The limitations of NAIMS are also discussed.

    View details for DOI 10.1021/acs.analchem.6b01212

    View details for Web of Science ID 000376223500071

    View details for PubMedID 27087600

  • High-Resolution Live-Cell Imaging and Analysis by Laser Desorption/Ionization Droplet Delivery Mass Spectrometry ANALYTICAL CHEMISTRY Lee, J. K., Jansson, E. T., Nam, H. G., Zare, R. N. 2016; 88 (10): 5453-5461

    Abstract

    We have developed a new ambient-ionization mass spectrometric technique named laser desorption/ionization droplet delivery mass spectrometry (LDIDD-MS). LDIDD-MS permitted high-resolution, high-sensitivity imaging of tissue samples as well as measurement of both single-cell apoptosis and live-cell exocytosis. A pulsed (15 Hz) UV laser beam (266 nm) was focused on a surface covered with target analytes to trigger their desorption and ionization. A spray of liquid droplets was simultaneously directed onto the laser-focused surface region to capture the ionized analytes and deliver them to a mass spectrometer. The approach of rapid and effective capturing molecules after laser desorption/ionization allowed the limit of detection for the amino acid lysine to be as low as 2 amol under ambient ionization conditions. Two-dimensional maps of the desorbed/ionized species were recorded by moving the sample on an XY translational stage. The spatial resolution for imaging with LDIDD-MS was determined to be 2.4 µm for an ink-printed pattern and 3 µm for mouse brain tissue. We applied LDIDD-MS to single-cell analysis of apoptotic HEK cells. Differences were observed in the profiles of fatty acids and lipids between healthy HEK cells and those undergoing apoptosis: We observed the upregulation of phosphatidylcholine (PC) with a relatively shorter carbon chain length and downregulation of PC with a relatively longer carbon chain length. We also applied LDIDD-MS for a real-time direct measurement of live-cell exocytosis. The catecholamine dopamine and trace amines (phenethylamine and tyramine) were detected from live PC12 cells without damaging them.

    View details for DOI 10.1021/acs.analchem.6b00881

    View details for Web of Science ID 000376223500059

    View details for PubMedID 27110027

  • Acceleration of reaction in charged microdroplets QUARTERLY REVIEWS OF BIOPHYSICS Lee, J. K., Banerjee, S., Nam, H. G., Zare, R. N. 2015; 48 (4): 437-444

    Abstract

    Using high-resolution mass spectrometry, we have studied the synthesis of isoquinoline in a charged electrospray droplet and the complexation between cytochrome c and maltose in a fused droplet to investigate the feasibility of droplets to drive reactions (both covalent and noncovalent interactions) at a faster rate than that observed in conventional bulk solution. In both the cases we found marked acceleration of reaction, by a factor of a million or more in the former and a factor of a thousand or more in the latter. We believe that carrying out reactions in microdroplets (about 1-15 μm in diameter corresponding to 0·5 pl - 2 nl) is a general method for increasing reaction rates. The mechanism is not presently established but droplet evaporation and droplet confinement of reagents appear to be two important factors among others. In the case of fused water droplets, evaporation has been shown to be almost negligible during the flight time from where droplet fusion occurs and the droplets enter the heated capillary inlet of the mass spectrometer. This suggests that (1) evaporation is not responsible for the acceleration process in aqueous droplet fusion and (2) the droplet-air interface may play a significant role in accelerating the reaction. We argue that this 'microdroplet chemistry' could be a remarkable alternative to accelerate slow and difficult reactions, and in conjunction with mass spectrometry, it may provide a new arena to study chemical and biochemical reactions in a confined environment.

    View details for DOI 10.1017/S0033583515000086

    View details for Web of Science ID 000364764300008

    View details for PubMedID 26537403

  • Microdroplet fusion mass spectrometry for fast reaction kinetics PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Lee, J. K., Kim, S., Nam, H. G., Zare, R. N. 2015; 112 (13): 3898-3903

    Abstract

    We investigated the fusion of high-speed liquid droplets as a way to record the kinetics of liquid-phase chemical reactions on the order of microseconds. Two streams of micrometer-size droplets collide with one another. The droplets that fused (13 μm in diameter) at the intersection of the two streams entered the heated capillary inlet of a mass spectrometer. The mass spectrum was recorded as a function of the distance x between the mass spectrometer inlet and the droplet fusion center. Fused droplet trajectories were imaged with a high-speed camera, revealing that the droplet fusion occurred approximately within a 500-μm radius from the droplet fusion center and both the size and the speed of the fused droplets remained relatively constant as they traveled from the droplet fusion center to the mass spectrometer inlet. Evidence is presented that the reaction effectively stops upon entering the heated inlet of the mass spectrometer. Thus, the reaction time was proportional to x and could be measured and manipulated by controlling the distance x. Kinetic studies were carried out in fused water droplets for acid-induced unfolding of cytochrome c and hydrogen-deuterium exchange in bradykinin. The kinetics of the former revealed the slowing of the unfolding rates at the early stage of the reaction within 50 μs. The hydrogen-deuterium exchange revealed the existence of two distinct populations with fast and slow exchange rates. These studies demonstrated the power of this technique to detect reaction intermediates in fused liquid droplets with microsecond temporal resolution.

    View details for DOI 10.1073/pnas.1503689112

    View details for Web of Science ID 000351914500040

    View details for PubMedID 25775573

  • High Resolution Mass Spectrometric Imaging for Single Cell Metabolic Analysis 58th Annual Meeting of the Biophysical-Society Lee, J. K., Kim, S., Nam, H. G., Zare, R. N. CELL PRESS. 2014: 798A–798A
  • Photo-Activated Crosslinking Mass Spectrometry for Studying Biomolecular Interactions 58th Annual Meeting of the Biophysical-Society Kim, S., Lee, J. K., Nam, H. G., Zare, R. N. CELL PRESS. 2014: 459A–459A
  • Ca2+ Dynamics in Apoptosis: Real-Time Data and Mathematical Modeling 56th Annual Meeting of the Biophysical-Society Lu, S., Lee, J. K., Madhukar, A. CELL PRESS. 2012: 628A–628A
  • Real-Time Dynamics of Ca2+, Phosphatidylserine, Caspase-3/7, and Morphological Changes in Apoptosis: Retinal Ganglion Cells Under Elevated Pressure 55th Annual Meeting of the Biophysical-Society Lu, S., Lee, J. K., Madhukar, A. CELL PRESS. 2011: 41–41
  • Real-Time Dynamics of Ca2+, Caspase-3/7, and Morphological Changes in Retinal Ganglion Cell Apoptosis under Elevated Pressure PLOS ONE Lee, J. K., Lu, S., Madhukar, A. 2010; 5 (10)

    Abstract

    Quantitative information on the dynamics of multiple molecular processes in individual live cells under controlled stress is central to the understanding of the cell behavior of interest and the establishment of reliable models. Here, the dynamics of the apoptosis regulator intracellular Ca(2+), apoptosis effector caspase-3/7, and morphological changes, as well as temporal correlation between them at the single cell level, are examined in retinal gangling cell line (differentiated RGC-5 cells) undergoing apoptosis at elevated hydrostatic pressure using a custom-designed imaging platform that allows long-term real-time simultaneous imaging of morphological and molecular-level physiological changes in large numbers of live cells (beyond the field-of-view of typical microscopy) under controlled hydrostatic pressure. This examination revealed intracellular Ca(2+) elevation with transient single or multiple peaks of less than 0.5 hour duration appearing at the early stages (typically less than 5 hours after the onset of 100 mmHg pressure) followed by gradual caspase-3/7 activation at late stages (typically later than 5 hours). The data reveal a strong temporal correlation between the Ca(2+) peak occurrence and morphological changes of neurite retraction and cell body shrinkage. This suggests that Ca(2+) elevation, through its impact on ion channel activity and water efflux, is likely responsible for the onset of apoptotic morphological changes. Moreover, the data show a significant cell-to-cell variation in the onset of caspase-3/7 activation, an inevitable consequence of the stochastic nature of the underlying biochemical reactions not captured by conventional assays based on population-averaged cellular responses. This real-time imaging study provides, for the first time, statistically significant data on simultaneous multiple molecular level changes to enable refinements and testing of models of the dynamics of mitochondria-mediated apoptosis. Further, the platform developed and the approach has direct significance to the study of a variety of signaling pathway phenomena.

    View details for DOI 10.1371/journal.pone.0013437

    View details for Web of Science ID 000283045300018

    View details for PubMedID 20976135

  • Surface modification of polydimethylsiloxane (PDMS) induced proliferation and neural-like cells differentiation of umbilical cord blood-derived mesenchymal stem cells JOURNAL OF MATERIALS SCIENCE-MATERIALS IN MEDICINE Kim, S., Lee, J. K., Kim, J. W., Jung, J., Seo, K., Park, S., Roh, K., Lee, S., Hong, Y. H., Kim, S. J., Lee, Y., Kim, S. J., Kang, K. 2008; 19 (8): 2953-2962

    Abstract

    Stem cell-based therapy has recently emerged for use in novel therapeutics for incurable diseases. For successful recovery from neurologic diseases, the most pivotal factor is differentiation and directed neuronal cell growth. In this study, we fabricated three different widths of a micro-pattern on polydimethylsiloxane (PDMS; 1, 2, and 4 microm). Surface modification of the PDMS was investigated for its capacity to manage proliferation and differentiation of neural-like cells from umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs). Among the micro-patterned PDMS fabrications, the 1 microm-patterned PDMS significantly increased cell proliferation and most of the cells differentiated into neuronal cells. In addition, the 1 microm-patterned PDMS induced an increase in cytosolic calcium, while the differentiated cells on the flat and 4 microm-patterned PDMS had no response. PDMS with a 1 microm pattern was also aligned to direct orientation within 10 degrees angles. Taken together, micro-patterned PDMS supported UCB-MSC proliferation and induced neural like-cell differentiation. Our data suggest that micro-patterned PDMS might be a guiding method for stem cell therapy that would improve its therapeutic action in neurological diseases.

    View details for DOI 10.1007/s10856-008-3413-6

    View details for Web of Science ID 000256476600023

    View details for PubMedID 18360798

  • Neural prosthesis in the wake of nanotechnology: controlled growth of neurons using surface nanostructures 1st Congress of the International-Society-of-Reconstructive-Neurosurgical-Societies Lee, J. K., Baac, H., Song, S., Jang, E., Lee, S., Park, D., Kim, S. J. SPRINGER-VERLAG WIEN. 2006: 141–144

    Abstract

    Neural prosthesis has been successfully applied to patients with motional or sensory disabilities for clinical purpose. To enhance the performance of the neural prosthetic device, the electrodes for the biosignal recording or electrical stimulation should be located in closer proximity to target neurons than they are now. Instead of revising the prior implanting surgery to improve the electrical contact of neurons, we propose a technique that can bring the neurons closer to the electrode sites. A new method is investigated that can control the direction of neural cell growth using surface nanostructures. We successfully guide the neurons to the position of the microelectrodes by providing a surface topographical cue presented by the surface nanostructure on a photoresponsive polymer material. Because the surface structure formed by laser holography is reversible and repeatable, the geometrical positioning of the neurons to microelectrodes can be adjusted by applying laser treatment during the surgery for the purpose of improving the performance of neural prosthetic device.

    View details for Web of Science ID 000245597400027

    View details for PubMedID 17370781

  • Spatial Patterning of fibroblast cells with fabricating holographic patterning on the photoresponsive polymer 2005 27TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY, VOLS 1-7 Lee, J. K., Jang, E., Lee, S., Kim, S. J. 2005: 4107-4110

    Abstract

    Micro topographic feature was obtained by laser holographic fabrication on the photoresponsive polymer. Surface feature was localized using photomask for developing two dimensional cellular pattering. Fibroblast cells were cultured and proliferated only on the patterned substrate. Obtained cellular pattering suggests that the laser fabricating with photoresponsive polymer would be applied to regenerating new tissue and developing biomedical device of living cells.

    View details for Web of Science ID 000238998403197

    View details for PubMedID 17281136

  • Photo-triggering of the membrane gates in photo-responsive polymer for drug release 2005 27TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY, VOLS 1-7 Lee, J. K., Lee, H., Jang, E., Lee, S., Kim, S. J. 2005: 5069-5072

    Abstract

    The use of light stimulus for triggering drug is a promising method for accurate drug delivery. A new approach using azopolymer membrane and laser holography was investigated for developing light-triggering drug delivery system. Polymeric drug delivery system was prepared by covering azopolymer membrane on a drug agent. Holographic laser interference generated surface relief grating pattern on the azopolymer surface. The widths and depths of gates on the polymer membrane were easily modified by adjusting incident angle and irradiation time. Ar laser made the polymeric membrane permeable to the drug agent and release it in a solution. This result indicated that the azopolymer and laser holography would provide a strong foundation for the light-triggering drug delivery system.

    View details for Web of Science ID 000238998404158

    View details for PubMedID 17281386

  • The topographical guidance of neurons cultured on holographic photo-responsive polymer PROCEEDINGS OF THE 26TH ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY, VOLS 1-7 Lee, J. K., Baac, H., Song, S. H., Lee, S. D., Park, D., Kim, S. J. 2004; 26: 4970-4973