Henry Du - Academia.edu (original) (raw)
Papers by Henry Du
Langmuir, 2008
Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitr... more Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.
Journal of Nanoscience and Nanotechnology, 2008
A layer-by-layer (LbL) self-assembly strategy involving oppositely charged Ag nanoparticles was u... more A layer-by-layer (LbL) self-assembly strategy involving oppositely charged Ag nanoparticles was used to deposit a nanoshell of Ag nanoparticles on silica microspheres for trace chemical measurements in aqueous solutions by means of surface-enhanced Raman scattering (SERS). Positively charged Ag nanoparticles were produced by reduction of Ag nitrate in a solution mixture of branched polyethyleneimine (BPEI) and N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (HEPES) under UV irradiation whereas negatively charged Ag nanoparticles were synthesized by the conventional citrate reduction method. The density of Ag nanoparticles in the nanoshell exhibits a strong correlation with the layer number and the nanoparticle type. Thiocyanate (SCN-) and crystal violet and were used as model positively and negatively charged analytes respectively to assess the robustness of the resultant core-shell nanostructures for SERS measurements. High sensitivity, at ppt for crystal violet and ppb for SCN-, was obtained when the surface charge of the terminating Ag layer in the LbL self-assembled nanoshell was opposite to the ionic nature of the analyte of interest due to enhanced adsorption of the analyte to the Ag nanoparticles facilitated by strong electrostatic attraction. The microsphere-nanoshell structures were all individually SERS-active, making them excellent candidate platform for integration with microfluidic systems for in situ SERS-based sensing and measurements.
Journal of the American Chemical Society, 2009
Oxidation of the Ag nanoparticle surface has a dramatic effect on the adsorption, orientation, an... more Oxidation of the Ag nanoparticle surface has a dramatic effect on the adsorption, orientation, and SERS detection limit of nitroaromatic molecules in aqueous solutions. Ultrasensitive SERS detection of p-nitrophenol can be achieved when oxidation of surface-immobilized Ag nanoparticles is inhibited by replacing the oxygen dissolved in water with argon gas. The presence of silver oxide at the nanoparticle surface hinders charge transfer between the aromatic ring and the underlying Ag metal surface and drastically decreases the overall detection sensitivity.
Analytical Chemistry, 2011
b S Supporting Information S urface-enhanced Raman scattering (SERS) is an attractive analytical ... more b S Supporting Information S urface-enhanced Raman scattering (SERS) is an attractive analytical technique for label-free detection and identification of chemical and biological species at trace concentrations. 1À3 While significant advances have been made in engineering various SERSactive metal (mainly Ag and Au) nanostructures, 4À13 SERS is far from being adopted as a routine analytical technique. This is mainly due to the strong dependence of SERS enhancement on structural, morphological features of metal nanostructures 13À20 as well as on surface chemistry and details of molecular binding. Significant challenges remain, in particular, when Ag, often a metal of choice because of its inherently higher SERS activity compared to its Au counterpart, is used in preparation of SERS-active nanostructures. Silver is prone to oxidation even under ambient conditions, and this results in significant changes in the chemical and plasmonic properties of Ag nanoparticles (Ag NPs). 21À25 For example, exposure to air or oxygen has been reported to cause a red shift of Ag NP plasmonic bands 21,23,24 Another example is an interesting observation of the dependence of oxidation of Ag NPs on the NP size. In this report, we aim to identify and quantify surface oxidation species using X-ray photoelectron spectroscopy (XPS) and to demonstrate their effect on SERS activity of Ag NPs.
Langmuir, 2012
We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of nobl... more We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of noble metal nanostructures, has a dramatic effect on their surface-enhanced Raman scattering (SERS) activity and enables highly selective SERS detection of analytes of various type and charge. Nanostructures studied include PVP-stabilized Au-Ag nanoshells synthesized by galvanic exchange reaction of citrate-reduced Ag nanoparticles (NPs), as well as solid citrate-reduced Ag and Au NPs, both before and after stabilization with PVP. All nanostructures were characterized in terms of their size, surface plasmon resonance wavelength, surface charge, and chemical composition. While the SERS activities of the parent citrate-reduced Ag and Au NPs are similar for rhodamine 6G (R6G) and 1,2-bis(4-pyridyl)ethylene (BPE) at various pH values, PVP-stabilized nanostructures demonstrate large differences in SERS enhancement factors (EFs) between these analytes depending on their chemical nature and protonation state. At pH values higher than BPE's pK(a2) of 5.65, where the analyte is largely unprotonated, the PVP-coated Au-Ag nanoshells showed a high SERS EF of >10(8). In contrast, SERS EFs were 10(3)- to 10(5)-fold lower for the protonated form of BPE at lower pH values, or for the usually highly SERS-active cationic R6G. The differential SERS activity of PVP-stabilized nanostructures is a result of discriminatory binding of analytes within-adsorbed PVP monolayer and a subsequent increase of analyte concentration at the nanostructure surface. Our experimental and theoretical quantum chemical calculations show that BPE binding with PVP-stabilized Au-Ag nanoshells is stronger when the analyte is in its unprotonated form as compared to its cationic, protonated form at a lower pH.
In this paper we report a study of novel thermally stable surface enhanced Raman scattering (SERS... more In this paper we report a study of novel thermally stable surface enhanced Raman scattering (SERS) substrates consisting of Ag nanoparticles immobilized on an anodized aluminum oxide (AAO) support.
We report a study on polymer-mediated immobilization of non-aggregated Ag nanoparticles on planar... more We report a study on polymer-mediated immobilization of non-aggregated Ag nanoparticles on planar glass substrates and the resultant surface-enhanced Raman scattering (SERS) activity using Rhodamine 6G (R6G) as a model molecule. Ag colloidal solution with an average particle diameter of 70 nm was prepared by citrate reduction of AgNO 3 using the Lee-Meisel method, and subsequent fractionation by filtration. A self-assembled polyallylamine hydrochloride (PAH) monolayer was employed as the intermediate polymer layer. We have shown that the coverage density of Ag nanoparticles on the glass substrates correlates with the amount of adsorbed PAH. This parameter can be easily controlled by varying the pH and ionic strength during polymer deposition. The highest coverage density was obtained for the polymer deposition from buffer solutions at pH 9.0, which additionally contained 0.25 M NaCl. The SERS-active substrates were robust and stable in 0.5 M NaCl solutions, as well as under extreme acidic and basic conditions. The glass substrates with immobilized non-aggregated Ag nanoparticles exhibited SERS enhancement and provided in situ detection sensitivity of R6G at 5 ppt level, with estimated surface coverage of two to four R6G molecules per silver particle. We found that adsorption of R6G in the presence of N-(2-hydroxyethyl)piperazine-N -(2-ethanesulfonic acid) (HEPES) buffer resulted in complete inhibition of photodecomposition of adsorbed R6G molecules. Studies of the effect of sodium chloride on the SERS activity of the glass substrates with individually attached nanoparticles showed that chloride anions resulted in a two to threefold increase in SERS intensity. Our approach has enabled the isolation of chloride-activated SERS enhancement from contributions arising from nanoparticle aggregation. The observed SERS enhancement in salt solutions results from coadsorption of chloride ions by the silver surface, and manifests itself in changing of orientation of adsorbed R6G molecules. The observed enhancement was reversed by exposing the substrate to 514-nm laser radiation, which resulted in the cleavage of Ag-Cl bonds.
Langmuir, 2006
This study concerns the design of protein-resistant polymer adsorbed layers for the control of su... more This study concerns the design of protein-resistant polymer adsorbed layers for the control of surface binding of biospecific recognition entities. Polymer surface layers were prepared using the adsorption of poly(allylamine hydrochloride) (PAH), poly(L-lysine) (PL), and branched and linear polyethyleneimine (PEI) and further modified by the covalent attachment of biotin for specific avidin attachment. The adsorption of PAH, PL, and PEI on silicon substrates was studied as a function of pH and ionic strength using ellipsometry. Average dry layer thicknesses of ∼10, ∼5, ∼9, and ∼3 Å ((1 Å) were obtained when polymer adsorption occurred from solutions at pH 9.5 that contained 0.5 M NaCl for PAH, PL, branched PEI, and linear PEI, respectively. These polymers showed significant differences in their efficiency to suppress nonspecific avidin adsorption. At low ionic strength, avidin adsorption occurred on all polymer-coated surfaces at basic pH values, despite the same positive electrostatic charge for protein globules and the surface. Though the net electrostatic repulsion between avidin molecules and branched PEI was efficiently screened in a protein solution of pH 7 and 0.15 M NaCl, branched-PEI coatings of high molecular weight were unique in their ability to provide avidin-resistant surfaces as a result of steric hindrance from the branched architecture of adsorbed polymer chains. All polymers studied were effective in suppressing avidin adsorption at pH 3 as a result of protonation of the avidin surface functional groups at this pH. Branched-PEI-coated surfaces were also effective for the suppression of smaller positively charged proteins such as lysozyme and ribonuclease A at pH 7 and 0.15 M NaCl. They were also resistant to the adsorption of negatively charged proteins such as BSA and fibrinogen at pH 7 and 0.75 M NaCl. Furthermore, by using PEI-modified protein-repellent surfaces, selective binding of avidin was achieved to surface-bound silver nanoparticles, which should provide a promising application for the label-free detection of biological species using surface-enhanced Raman scattering (SERS). Terrettaz, S.; Michel, R.; Tirelli, N.; Vogel, H.; Textor, M.; Hubbell, J. A. Nat. Mater. 2003, 2, 259-264. (4) Dalsin, J. L.; Lin, L.; Tosatti, S.; Vörös, J.; Textor, M.; Messersmith, P. B. Langmuir 2005, 21, 640-646. (5) Holmberg, K.; Tiberg, F.; Malmsten, M.; Brink, C. Colloids Surf., A 1997, 123-124, 297-306. (6) Wei, J.; Ravn, D. B.; Gram, L.; Kingshott, P. Colloids Surf., B 2003, 32, 275-291.
Langmuir, 2008
Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitr... more Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.
Biosensors & Bioelectronics
Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this stru... more Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this structure with an optically aligned flow cell, we have developed an optofluidic refractive index transduction platform for label-free biosensing. The LPG-PCF scheme possesses extremely high sensitivity to the change in refractive index induced by localized binding event in different solution media. A model immunoassay experiment was carried out inside the air channels of PCF by a series of surface modification steps in sequence that include adsorption of poly(allylamine hydrochloride) monolayer, immobilization of anti-rat bone sialoprotein monoclonal primary antibody, and binding interactions with non-specific goat anti-rabbit IgG (H+L) and specific secondary goat anti-mouse IgG (H+L) antibodies. These adsorption and binding events were monitored in situ using the LPG-PCF by measuring the shift of the core-to-cladding mode coupling resonance wavelength. Steady and significant resonance chang...
ABSTRACT Au nanostructures that exhibit strong localized surface plasmon resonance (SPR) have exc... more ABSTRACT Au nanostructures that exhibit strong localized surface plasmon resonance (SPR) have excellent potential for photo-medicine, among a host of other applications. Here, we report the synthesis and use of colloidal gold nanorings (GNRs) with potential for enhanced photodynamic therapy of cancer. The GNRs were fabricated via galvanic replacement reaction of sacrificial Co nanoparticles in gold salt solution with low molecular weight (Mw = 2,500) poly(vinylpyrrolidone) (PVP) as a stabilizing agent. The size and the opening of the GNRs were controlled by the size of the starting Co particles and the concentration of the gold salt. UV-Vis absorption measurements indicated the tunability of the SPR of the GNRs from 560 nm to 780 nm. MTT assay showed that GNRs were non-toxic and biocompatible when incubated with breast cancer cells as well as the healthy counterpart cells. GNRs conjugated with 5-aminolevulinic acid (5-ALA) photosensitizer precursor led to elevated formation of reactive oxygen species and improved efficacy of photodynamic therapy of breast cancer cells under light irradiation compared to 5-ALA alone. These results can be attributed to significantly enhance localized electromagnetic field of the GNRs.
Biomedical Optics Express, 2015
We report an early, noninvasive and rapid prognostic method of predicting potential acute kidney ... more We report an early, noninvasive and rapid prognostic method of predicting potential acute kidney dysfunction using surface-enhanced Raman scattering (SERS). Our analysis was performed on urine samples collected prospectively from 58 kidney transplant patients using a He-Ne laser (632.8 nm) as the excitation source. All abnormal kidney function episodes (three acute rejections and two acute kidney failures that were eventually diagnosed independently by clinical biopsy) consistently exhibited unique SERS spectral features in just one day following the transplant surgery. These results suggested that SERS analysis provides an early and more specific indication to kidney function than the clinically used biomarker, serum creatinine (sCr).
The unique feature of photonic crystal fiber (PCF) both as a light guide and a liquid transmissio... more The unique feature of photonic crystal fiber (PCF) both as a light guide and a liquid transmission cell allows synergistic integration of optics and microfluidics to form an unconventional optofluidic platform of long interaction path limited only by the fiber length. We report the strategy and methods in realizing full-length surface-enhanced Raman scattering (SERS) PCF optofluidics by immobilization of negatively
Optics Letters, 2006
We show that solid-core photonic crystal fiber (PCF) is a promising platform for evanescent-field... more We show that solid-core photonic crystal fiber (PCF) is a promising platform for evanescent-field Raman spectroscopy of low-volume analytes. The Raman peak ratio of a silica core as a background to acetonitrile solution as analyte contained in the air holes maintains a constant value despite varying laser power and fiber length in a set of measurements. The Raman signal from the silica core can be used to eliminate the need to account for the coupling losses. These results demonstrate the feasibility of quantitative measurements using PCF as a Raman platform with silica as an internal reference. In addition, integrated Raman intensity increases with the length of the PCF due to long path length of light.
Optics Letters, 2010
We report numerical simulation and hyperspectral Raman imaging of three index-guiding solid-core ... more We report numerical simulation and hyperspectral Raman imaging of three index-guiding solid-core photonic crystal fibers (PCFs) of different air-cladding microstructures to assess their respective potential for evanescent-field Raman spectroscopy, with an emphasis on achieving surface-enhanced Raman scattering (SERS) over the entire fiber length. Suspended-core PCF consisting of a silica core surrounded by three large air channels conjoined by a thin silica web is the most robust of the three SERS-active PCFs, with a demonstrated detection sensitivity of 1x10(-10) M R6G in an aqueous solution of only approximately 7.3 microL sampling volume.
Optics Letters, 2009
A 30 cm long solid-core photonic crystal fiber (PCF) with immobilized and discrete Ag nanoparticl... more A 30 cm long solid-core photonic crystal fiber (PCF) with immobilized and discrete Ag nanoparticles was used to obtain forward-propagating surface-enhanced Raman scattering (SERS) of 2 microM Rhodamine 6G (R6G) aqueous solution filled in the cladding air channels. The intensity distributions of characteristic Raman vibrational bands of silica and R6G in PCF were mapped for the first time to our knowledge by hyperspectral Raman imaging. We show that the measured SERS intensity arises exclusively from the forward-propagating core mode as a result of evanescent-field interaction with R6G in the innermost ring of the cladding air channels.
Optics Express, 2007
We report the spectral characteristics of CO 2 -laser inscribed longperiod gratings (LPGs) in end... more We report the spectral characteristics of CO 2 -laser inscribed longperiod gratings (LPGs) in endlessly single mode photonic crystal fiber (PCF) subject to macro-bending. The coupling modes as a result of bending were studied by examining the shifts of resonant wavelengths, the splits of attenuation bands, and the variation in coupling strength of the transmission spectra. A bending coefficient of ~ 27.9 nm·m was determined in the PCF at 180 0 rotational orientation relative to the point of laser inscription in the curvature range from 2.6 m -1 to 3.5 m -1 . Compared with conventional fiber LPGs fabricated using the same method, the PCF-based LPGs possess higher sensitivity both to bending and orientation, making them promising for sensor applications.
Optics express, Jan 24, 2008
Light refraction at the planar boundary of dielectric media prevents light propagation in the hig... more Light refraction at the planar boundary of dielectric media prevents light propagation in the higher refractive index medium at angles beyond the critical value. This limitation is lifted when the evanescent wave is excited at the lower refractive index side of the interface. In this work we quantify polarization and angle dependence of surface-enhanced Raman scattering (SERS) intensity beyond the critical angle. Specifically, Raman spectra of thiocyanate molecules adsorbed on clustered silver nanoparticles at the water-glass interface were acquired using evanescent excitation and detection. Detected SERS signal polarization and scattering angle dependence are shown to be in agreement with a simple model based on excitation and radiation of a classical dipole near a lossless interface.
Optical Engineering, 2008
... and surface-enhanced Raman scattering. [Optical Engineering 47, 040502 (2008)]. Yun Han, memb... more ... and surface-enhanced Raman scattering. [Optical Engineering 47, 040502 (2008)]. Yun Han, member spie, Maung Kyaw Khaing Oo, Yinian Zhu, Limin Xiao, M. Suleyman Demohan, Wei Jin, Henry Du, member spie. Abstract. ...
Nature, 1996
TITANIUM occurs widely in the Earth's crust, traces of it being present in most rocks, soils... more TITANIUM occurs widely in the Earth's crust, traces of it being present in most rocks, soils and clays. It is estimated that titanosilicate minerals alone number more than 100; but, remarkably, in only one of these does the Ti(IV) ion take up fivefold coordination. This is in fresnoite1 (Ba2TiSi2O8), which contains square-pyramidal TiO5 polyhedra. In the course of a programme2-11 to produce new microporous and mesoporous solid catalysts, we have discovered an unusual non-centrosymmetric, tetragonal layered solid (Na4Ti2Si8O22.4H2O), designated JDF-L1, which promises to have interesting applications in materials chemistry. This material contains five-coordinate Ti(IV) ions in the form of TiO5 square pyramids in which each of the vertices of the base is linked to SiO4 tetrahedra [TiO.O4(SiO3)4] to form continuous sheets. The structure was solved by applying ab initio methods to data obtained by X-ray absorption spectroscopy and powder X-ray diffraction. The interlamellar Na+ ions of JDF-L1 are replaceable by protonated amines, and after treatment with a mixture of dilute acid and hydrogen peroxide the parent solid selectively oxidizes phenol to quinone. These results indicate that the material should have useful catalytic, intercalation and ion-exchange properties analogous to those of aluminosilicate clays.
Langmuir, 2008
Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitr... more Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.
Journal of Nanoscience and Nanotechnology, 2008
A layer-by-layer (LbL) self-assembly strategy involving oppositely charged Ag nanoparticles was u... more A layer-by-layer (LbL) self-assembly strategy involving oppositely charged Ag nanoparticles was used to deposit a nanoshell of Ag nanoparticles on silica microspheres for trace chemical measurements in aqueous solutions by means of surface-enhanced Raman scattering (SERS). Positively charged Ag nanoparticles were produced by reduction of Ag nitrate in a solution mixture of branched polyethyleneimine (BPEI) and N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (HEPES) under UV irradiation whereas negatively charged Ag nanoparticles were synthesized by the conventional citrate reduction method. The density of Ag nanoparticles in the nanoshell exhibits a strong correlation with the layer number and the nanoparticle type. Thiocyanate (SCN-) and crystal violet and were used as model positively and negatively charged analytes respectively to assess the robustness of the resultant core-shell nanostructures for SERS measurements. High sensitivity, at ppt for crystal violet and ppb for SCN-, was obtained when the surface charge of the terminating Ag layer in the LbL self-assembled nanoshell was opposite to the ionic nature of the analyte of interest due to enhanced adsorption of the analyte to the Ag nanoparticles facilitated by strong electrostatic attraction. The microsphere-nanoshell structures were all individually SERS-active, making them excellent candidate platform for integration with microfluidic systems for in situ SERS-based sensing and measurements.
Journal of the American Chemical Society, 2009
Oxidation of the Ag nanoparticle surface has a dramatic effect on the adsorption, orientation, an... more Oxidation of the Ag nanoparticle surface has a dramatic effect on the adsorption, orientation, and SERS detection limit of nitroaromatic molecules in aqueous solutions. Ultrasensitive SERS detection of p-nitrophenol can be achieved when oxidation of surface-immobilized Ag nanoparticles is inhibited by replacing the oxygen dissolved in water with argon gas. The presence of silver oxide at the nanoparticle surface hinders charge transfer between the aromatic ring and the underlying Ag metal surface and drastically decreases the overall detection sensitivity.
Analytical Chemistry, 2011
b S Supporting Information S urface-enhanced Raman scattering (SERS) is an attractive analytical ... more b S Supporting Information S urface-enhanced Raman scattering (SERS) is an attractive analytical technique for label-free detection and identification of chemical and biological species at trace concentrations. 1À3 While significant advances have been made in engineering various SERSactive metal (mainly Ag and Au) nanostructures, 4À13 SERS is far from being adopted as a routine analytical technique. This is mainly due to the strong dependence of SERS enhancement on structural, morphological features of metal nanostructures 13À20 as well as on surface chemistry and details of molecular binding. Significant challenges remain, in particular, when Ag, often a metal of choice because of its inherently higher SERS activity compared to its Au counterpart, is used in preparation of SERS-active nanostructures. Silver is prone to oxidation even under ambient conditions, and this results in significant changes in the chemical and plasmonic properties of Ag nanoparticles (Ag NPs). 21À25 For example, exposure to air or oxygen has been reported to cause a red shift of Ag NP plasmonic bands 21,23,24 Another example is an interesting observation of the dependence of oxidation of Ag NPs on the NP size. In this report, we aim to identify and quantify surface oxidation species using X-ray photoelectron spectroscopy (XPS) and to demonstrate their effect on SERS activity of Ag NPs.
Langmuir, 2012
We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of nobl... more We report that poly(vinylpyrrolidone) (PVP), a common stabilizer of colloidal dispersions of noble metal nanostructures, has a dramatic effect on their surface-enhanced Raman scattering (SERS) activity and enables highly selective SERS detection of analytes of various type and charge. Nanostructures studied include PVP-stabilized Au-Ag nanoshells synthesized by galvanic exchange reaction of citrate-reduced Ag nanoparticles (NPs), as well as solid citrate-reduced Ag and Au NPs, both before and after stabilization with PVP. All nanostructures were characterized in terms of their size, surface plasmon resonance wavelength, surface charge, and chemical composition. While the SERS activities of the parent citrate-reduced Ag and Au NPs are similar for rhodamine 6G (R6G) and 1,2-bis(4-pyridyl)ethylene (BPE) at various pH values, PVP-stabilized nanostructures demonstrate large differences in SERS enhancement factors (EFs) between these analytes depending on their chemical nature and protonation state. At pH values higher than BPE's pK(a2) of 5.65, where the analyte is largely unprotonated, the PVP-coated Au-Ag nanoshells showed a high SERS EF of >10(8). In contrast, SERS EFs were 10(3)- to 10(5)-fold lower for the protonated form of BPE at lower pH values, or for the usually highly SERS-active cationic R6G. The differential SERS activity of PVP-stabilized nanostructures is a result of discriminatory binding of analytes within-adsorbed PVP monolayer and a subsequent increase of analyte concentration at the nanostructure surface. Our experimental and theoretical quantum chemical calculations show that BPE binding with PVP-stabilized Au-Ag nanoshells is stronger when the analyte is in its unprotonated form as compared to its cationic, protonated form at a lower pH.
In this paper we report a study of novel thermally stable surface enhanced Raman scattering (SERS... more In this paper we report a study of novel thermally stable surface enhanced Raman scattering (SERS) substrates consisting of Ag nanoparticles immobilized on an anodized aluminum oxide (AAO) support.
We report a study on polymer-mediated immobilization of non-aggregated Ag nanoparticles on planar... more We report a study on polymer-mediated immobilization of non-aggregated Ag nanoparticles on planar glass substrates and the resultant surface-enhanced Raman scattering (SERS) activity using Rhodamine 6G (R6G) as a model molecule. Ag colloidal solution with an average particle diameter of 70 nm was prepared by citrate reduction of AgNO 3 using the Lee-Meisel method, and subsequent fractionation by filtration. A self-assembled polyallylamine hydrochloride (PAH) monolayer was employed as the intermediate polymer layer. We have shown that the coverage density of Ag nanoparticles on the glass substrates correlates with the amount of adsorbed PAH. This parameter can be easily controlled by varying the pH and ionic strength during polymer deposition. The highest coverage density was obtained for the polymer deposition from buffer solutions at pH 9.0, which additionally contained 0.25 M NaCl. The SERS-active substrates were robust and stable in 0.5 M NaCl solutions, as well as under extreme acidic and basic conditions. The glass substrates with immobilized non-aggregated Ag nanoparticles exhibited SERS enhancement and provided in situ detection sensitivity of R6G at 5 ppt level, with estimated surface coverage of two to four R6G molecules per silver particle. We found that adsorption of R6G in the presence of N-(2-hydroxyethyl)piperazine-N -(2-ethanesulfonic acid) (HEPES) buffer resulted in complete inhibition of photodecomposition of adsorbed R6G molecules. Studies of the effect of sodium chloride on the SERS activity of the glass substrates with individually attached nanoparticles showed that chloride anions resulted in a two to threefold increase in SERS intensity. Our approach has enabled the isolation of chloride-activated SERS enhancement from contributions arising from nanoparticle aggregation. The observed SERS enhancement in salt solutions results from coadsorption of chloride ions by the silver surface, and manifests itself in changing of orientation of adsorbed R6G molecules. The observed enhancement was reversed by exposing the substrate to 514-nm laser radiation, which resulted in the cleavage of Ag-Cl bonds.
Langmuir, 2006
This study concerns the design of protein-resistant polymer adsorbed layers for the control of su... more This study concerns the design of protein-resistant polymer adsorbed layers for the control of surface binding of biospecific recognition entities. Polymer surface layers were prepared using the adsorption of poly(allylamine hydrochloride) (PAH), poly(L-lysine) (PL), and branched and linear polyethyleneimine (PEI) and further modified by the covalent attachment of biotin for specific avidin attachment. The adsorption of PAH, PL, and PEI on silicon substrates was studied as a function of pH and ionic strength using ellipsometry. Average dry layer thicknesses of ∼10, ∼5, ∼9, and ∼3 Å ((1 Å) were obtained when polymer adsorption occurred from solutions at pH 9.5 that contained 0.5 M NaCl for PAH, PL, branched PEI, and linear PEI, respectively. These polymers showed significant differences in their efficiency to suppress nonspecific avidin adsorption. At low ionic strength, avidin adsorption occurred on all polymer-coated surfaces at basic pH values, despite the same positive electrostatic charge for protein globules and the surface. Though the net electrostatic repulsion between avidin molecules and branched PEI was efficiently screened in a protein solution of pH 7 and 0.15 M NaCl, branched-PEI coatings of high molecular weight were unique in their ability to provide avidin-resistant surfaces as a result of steric hindrance from the branched architecture of adsorbed polymer chains. All polymers studied were effective in suppressing avidin adsorption at pH 3 as a result of protonation of the avidin surface functional groups at this pH. Branched-PEI-coated surfaces were also effective for the suppression of smaller positively charged proteins such as lysozyme and ribonuclease A at pH 7 and 0.15 M NaCl. They were also resistant to the adsorption of negatively charged proteins such as BSA and fibrinogen at pH 7 and 0.75 M NaCl. Furthermore, by using PEI-modified protein-repellent surfaces, selective binding of avidin was achieved to surface-bound silver nanoparticles, which should provide a promising application for the label-free detection of biological species using surface-enhanced Raman scattering (SERS). Terrettaz, S.; Michel, R.; Tirelli, N.; Vogel, H.; Textor, M.; Hubbell, J. A. Nat. Mater. 2003, 2, 259-264. (4) Dalsin, J. L.; Lin, L.; Tosatti, S.; Vörös, J.; Textor, M.; Messersmith, P. B. Langmuir 2005, 21, 640-646. (5) Holmberg, K.; Tiberg, F.; Malmsten, M.; Brink, C. Colloids Surf., A 1997, 123-124, 297-306. (6) Wei, J.; Ravn, D. B.; Gram, L.; Kingshott, P. Colloids Surf., B 2003, 32, 275-291.
Langmuir, 2008
Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitr... more Positively charged silver nanoparticles, Ag [+], obtained by UV-assisted reduction of silver nitrate using branched poly(ethyleneimine) (BPEI) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) solutions as reducing agents, were immobilized on glass surfaces to produce substrates active in surface-enhanced Raman scattering (SERS). Negatively charged silver nanoparticles, Ag [-], synthesized via a modified citrate reduction method, were also investigated for comparison. At a sparse surface coverage of 30 nanoparticles/microm(2), substrates with immobilized Ag [+] showed increasing SERS sensitivity to a variety of anions in water in the order SO(4)(2-) < CN(-) < SCN(-) approximately ClO(4)(-), with corresponding binding constants of 10(5), 3.3 x 10(5), and 10(7) (for both SCN- and ClO(4)(-)) M(-1), respectively. This order followed the Hofmeister series of anion binding in water. Significantly, substrates with Ag [+] allowed limit of detection values of 8.0 x 10(-8) M (8 ppb) and 2.7 x 10(-7) M (7 ppb) for environmentally relevant perchlorate (ClO(4)(-)) and cyanide (CN(-)) anions, respectively. In contrast, substrates with immobilized Ag [-], even upon subsequent modification by a monolayer of BPEI for positive surface charge of the nanoparticles, showed a drastically lower sensitivity to these anions. The high sensitivity of substrates with Ag [+] for anion detection can be attributed to the presence of two types of functional groups, amino and amide, on the nanoparticle surface resulting from UV-assisted fragmentation of BPEI chains. Both amino and amide provide strong binding of anions with Ag [+] nanoparticles due to the synergistic effect through a combination of electrostatic, hydrogen bonding, and dispersive interactions.
Biosensors & Bioelectronics
Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this stru... more Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this structure with an optically aligned flow cell, we have developed an optofluidic refractive index transduction platform for label-free biosensing. The LPG-PCF scheme possesses extremely high sensitivity to the change in refractive index induced by localized binding event in different solution media. A model immunoassay experiment was carried out inside the air channels of PCF by a series of surface modification steps in sequence that include adsorption of poly(allylamine hydrochloride) monolayer, immobilization of anti-rat bone sialoprotein monoclonal primary antibody, and binding interactions with non-specific goat anti-rabbit IgG (H+L) and specific secondary goat anti-mouse IgG (H+L) antibodies. These adsorption and binding events were monitored in situ using the LPG-PCF by measuring the shift of the core-to-cladding mode coupling resonance wavelength. Steady and significant resonance chang...
ABSTRACT Au nanostructures that exhibit strong localized surface plasmon resonance (SPR) have exc... more ABSTRACT Au nanostructures that exhibit strong localized surface plasmon resonance (SPR) have excellent potential for photo-medicine, among a host of other applications. Here, we report the synthesis and use of colloidal gold nanorings (GNRs) with potential for enhanced photodynamic therapy of cancer. The GNRs were fabricated via galvanic replacement reaction of sacrificial Co nanoparticles in gold salt solution with low molecular weight (Mw = 2,500) poly(vinylpyrrolidone) (PVP) as a stabilizing agent. The size and the opening of the GNRs were controlled by the size of the starting Co particles and the concentration of the gold salt. UV-Vis absorption measurements indicated the tunability of the SPR of the GNRs from 560 nm to 780 nm. MTT assay showed that GNRs were non-toxic and biocompatible when incubated with breast cancer cells as well as the healthy counterpart cells. GNRs conjugated with 5-aminolevulinic acid (5-ALA) photosensitizer precursor led to elevated formation of reactive oxygen species and improved efficacy of photodynamic therapy of breast cancer cells under light irradiation compared to 5-ALA alone. These results can be attributed to significantly enhance localized electromagnetic field of the GNRs.
Biomedical Optics Express, 2015
We report an early, noninvasive and rapid prognostic method of predicting potential acute kidney ... more We report an early, noninvasive and rapid prognostic method of predicting potential acute kidney dysfunction using surface-enhanced Raman scattering (SERS). Our analysis was performed on urine samples collected prospectively from 58 kidney transplant patients using a He-Ne laser (632.8 nm) as the excitation source. All abnormal kidney function episodes (three acute rejections and two acute kidney failures that were eventually diagnosed independently by clinical biopsy) consistently exhibited unique SERS spectral features in just one day following the transplant surgery. These results suggested that SERS analysis provides an early and more specific indication to kidney function than the clinically used biomarker, serum creatinine (sCr).
The unique feature of photonic crystal fiber (PCF) both as a light guide and a liquid transmissio... more The unique feature of photonic crystal fiber (PCF) both as a light guide and a liquid transmission cell allows synergistic integration of optics and microfluidics to form an unconventional optofluidic platform of long interaction path limited only by the fiber length. We report the strategy and methods in realizing full-length surface-enhanced Raman scattering (SERS) PCF optofluidics by immobilization of negatively
Optics Letters, 2006
We show that solid-core photonic crystal fiber (PCF) is a promising platform for evanescent-field... more We show that solid-core photonic crystal fiber (PCF) is a promising platform for evanescent-field Raman spectroscopy of low-volume analytes. The Raman peak ratio of a silica core as a background to acetonitrile solution as analyte contained in the air holes maintains a constant value despite varying laser power and fiber length in a set of measurements. The Raman signal from the silica core can be used to eliminate the need to account for the coupling losses. These results demonstrate the feasibility of quantitative measurements using PCF as a Raman platform with silica as an internal reference. In addition, integrated Raman intensity increases with the length of the PCF due to long path length of light.
Optics Letters, 2010
We report numerical simulation and hyperspectral Raman imaging of three index-guiding solid-core ... more We report numerical simulation and hyperspectral Raman imaging of three index-guiding solid-core photonic crystal fibers (PCFs) of different air-cladding microstructures to assess their respective potential for evanescent-field Raman spectroscopy, with an emphasis on achieving surface-enhanced Raman scattering (SERS) over the entire fiber length. Suspended-core PCF consisting of a silica core surrounded by three large air channels conjoined by a thin silica web is the most robust of the three SERS-active PCFs, with a demonstrated detection sensitivity of 1x10(-10) M R6G in an aqueous solution of only approximately 7.3 microL sampling volume.
Optics Letters, 2009
A 30 cm long solid-core photonic crystal fiber (PCF) with immobilized and discrete Ag nanoparticl... more A 30 cm long solid-core photonic crystal fiber (PCF) with immobilized and discrete Ag nanoparticles was used to obtain forward-propagating surface-enhanced Raman scattering (SERS) of 2 microM Rhodamine 6G (R6G) aqueous solution filled in the cladding air channels. The intensity distributions of characteristic Raman vibrational bands of silica and R6G in PCF were mapped for the first time to our knowledge by hyperspectral Raman imaging. We show that the measured SERS intensity arises exclusively from the forward-propagating core mode as a result of evanescent-field interaction with R6G in the innermost ring of the cladding air channels.
Optics Express, 2007
We report the spectral characteristics of CO 2 -laser inscribed longperiod gratings (LPGs) in end... more We report the spectral characteristics of CO 2 -laser inscribed longperiod gratings (LPGs) in endlessly single mode photonic crystal fiber (PCF) subject to macro-bending. The coupling modes as a result of bending were studied by examining the shifts of resonant wavelengths, the splits of attenuation bands, and the variation in coupling strength of the transmission spectra. A bending coefficient of ~ 27.9 nm·m was determined in the PCF at 180 0 rotational orientation relative to the point of laser inscription in the curvature range from 2.6 m -1 to 3.5 m -1 . Compared with conventional fiber LPGs fabricated using the same method, the PCF-based LPGs possess higher sensitivity both to bending and orientation, making them promising for sensor applications.
Optics express, Jan 24, 2008
Light refraction at the planar boundary of dielectric media prevents light propagation in the hig... more Light refraction at the planar boundary of dielectric media prevents light propagation in the higher refractive index medium at angles beyond the critical value. This limitation is lifted when the evanescent wave is excited at the lower refractive index side of the interface. In this work we quantify polarization and angle dependence of surface-enhanced Raman scattering (SERS) intensity beyond the critical angle. Specifically, Raman spectra of thiocyanate molecules adsorbed on clustered silver nanoparticles at the water-glass interface were acquired using evanescent excitation and detection. Detected SERS signal polarization and scattering angle dependence are shown to be in agreement with a simple model based on excitation and radiation of a classical dipole near a lossless interface.
Optical Engineering, 2008
... and surface-enhanced Raman scattering. [Optical Engineering 47, 040502 (2008)]. Yun Han, memb... more ... and surface-enhanced Raman scattering. [Optical Engineering 47, 040502 (2008)]. Yun Han, member spie, Maung Kyaw Khaing Oo, Yinian Zhu, Limin Xiao, M. Suleyman Demohan, Wei Jin, Henry Du, member spie. Abstract. ...
Nature, 1996
TITANIUM occurs widely in the Earth's crust, traces of it being present in most rocks, soils... more TITANIUM occurs widely in the Earth's crust, traces of it being present in most rocks, soils and clays. It is estimated that titanosilicate minerals alone number more than 100; but, remarkably, in only one of these does the Ti(IV) ion take up fivefold coordination. This is in fresnoite1 (Ba2TiSi2O8), which contains square-pyramidal TiO5 polyhedra. In the course of a programme2-11 to produce new microporous and mesoporous solid catalysts, we have discovered an unusual non-centrosymmetric, tetragonal layered solid (Na4Ti2Si8O22.4H2O), designated JDF-L1, which promises to have interesting applications in materials chemistry. This material contains five-coordinate Ti(IV) ions in the form of TiO5 square pyramids in which each of the vertices of the base is linked to SiO4 tetrahedra [TiO.O4(SiO3)4] to form continuous sheets. The structure was solved by applying ab initio methods to data obtained by X-ray absorption spectroscopy and powder X-ray diffraction. The interlamellar Na+ ions of JDF-L1 are replaceable by protonated amines, and after treatment with a mixture of dilute acid and hydrogen peroxide the parent solid selectively oxidizes phenol to quinone. These results indicate that the material should have useful catalytic, intercalation and ion-exchange properties analogous to those of aluminosilicate clays.