We are interested in the physics of laser-material interactions, especially that of laser-induced processes for analytical and biomedical applications. Our aim is to develop novel optical solutions for ultra-trace analysis and bio-molecular studies. Two current activities are described below.
Analytical spectroscopy of laser plumes
When a sample target is ablated by a laser pulse, the vapor plume can be analyzed spectroscopically for composition determination. We pioneered two optical techniques for plume analysis that can detect trace amounts of multi elements simultaneously. One is by means of resonance-enhanced plasma spectroscopy. It minimizes the continuum background associated with laser induced plasmas. Relative to laser-induced breakdown spectroscopy, the signal-to-noise ratio is improved by orders of magnitude, thus allowing the quantitation of sodium and potassium at the single blood cell level. The other technique utilizes laser-excited atomic fluorescence. It has been traditionally handicapped by its one wavelength–one transition specificity. We showed, however, that numerous elements could be induced to fluoresce at a single excitation wavelength of 193 nm provided that the analytes were imbedded in dense plumes, such as those produced by pulsed laser ablation. This method eliminates the continuum plasma background and sub-ppb sensitivity was demonstrated in the analysis of aqueous lead colloids.
In total internal reflection, the evanescent wave induces fluorophores at the interface to emit while those in the bulk solution remain dark. It therefore singles out interface events. We use the technique to study the nonspecific adsorption of proteins at interfaces, at the ensemble as well as single molecule levels. Heterogeneities among pure protein species and protein-protein interactions were revealed with unprecedented details. We also investigate the chain of biochemical reactions starting with the specific binding of surface-immobilized protein receptors with their hormone ligands. We aim at elucidating the very rich kinetics of agonistic versus antagonistic bindings, down to single molecule resolution.
P.C. Chu, W.L. Yip, Y. Cai, and N.H. Cheung, “Multi-element analysis of ceramic and polymeric samples by ArF laser excited atomic fluorescence of ablated plumes”, J. Anal. At. Spectrom., 26, 1210-1216 (2011).
K.-C. Kwok and N.-H. Cheung, “Measuring binding kinetics of ligands with tethered receptors by fluorescence polarization and total internal reflection fluorescence”, Anal. Chem. 82, 3819-3825 (2010).
J.-Y. Chen, Y.-M. Lee, D. Zhao, N.-K. Mak, R.N.-S. Wong, W.-H. Chan, and N.-H. Cheung, “Quantum dot mediated photoproduction of reactive oxygen species for cancer cell annihilation”, Photochem. Photobiol. 82, 431-437 (2010).
K. Lau and N.H. Cheung, “Minimally Destructive and Multi-Element Analysis of Steel Alloys by Argon Fluoride Laser-Induced Plume Emissions”, Appl. Spectrosc., 63, 835-838 (2009).
W.L. Yip and N.H. Cheung, “Analysis of aluminum alloys by resonance-enhanced laser-induced breakdown spectroscopy: How the beam profile of the ablation laser and the energy of the dye laser affect analytical performance”, Spectrochim. Acta B, 64, 315-322 (2009).
K.M. Yeung, Z.J. Lu, and N.H. Cheung, “Adsorption of bovine serum albumin on fused silica: Elucidation of protein-protein interactions by single-molecule fluorescence microscopy”, Colloids and Surfaces B: Biointerfaces, 69, 246-250 (2009).
N.H. Cheung, “Spectroscopy of laser plumes for atto-mole and ng/g elemental analysis”, Appl. Spectrosc. Rev. 42, 235-250 (2007).
K.C. Kwok, K.M. Yeung, and N.H. Cheung, “Adsorption Kinetics of Bovine Serum Albumin on Fused Silica: Population Heterogeneities Revealed by Single-Molecule Fluorescence Microscopy”, Langmuir, 23, 1948-1952 (2007).
S.K. Ho and N.H. Cheung, “Minimally destructive and multi-element analysis of aluminum alloys by ArF laser-induced atomic emissions”, J. Anal. At. Spectrom., 22, 292-297 (2007).
S. K. Ho and N.H. Cheung, "Sensitive elemental analysis by ArF laser-induced fluorescence of laser ablation plumes: Elucidation of the fluorescence mechanism", Appl. Phys. Lett. 87, 264104 (2005).
S.L. Lui and N.H. Cheung, “Minimally destructive analysis of aluminum alloys using resonance-enhanced laser-induced plasma spectroscopy”, Anal. Chem., 77, 2617-2623 (2005).
S.K. Ho and N.H. Cheung, “Sub-Part-Per-Billion Analysis of Aqueous Lead Colloids by ArF Laser Induced Atomic Fluorescence”, Anal. Chem., 77, 193-199 (2005).
X.Y. Pu, W.Y. Ma, and N.H. Cheung, “Sensitive elemental analysis of aqueous colloids by laser-induced plasma spectroscopy”, Appl. Phys. Lett., 83, 3416-3418 (2003).