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Light scattering is a universal matter property that is especially prominent in nanoscale or larger materials. However, the effects of scattering-based cascading optical processes on experimental quantification of sample absorption, scattering, and emission intensities, as well as scattering and emission depolarization, have not been adequately addressed. Using a series of polystyrene nanoparticles (PSNPs) of different sizes as model analytes, we present a computational and experimental study on the effects of cascading light scattering on experimental quantification of NP scattering activities (scattering cross-section or molar coefficient), intensity, and depolarization. Part II and Part III of this series of companion articles explore the effects of cascading optical processes on sample absorption and fluorescence measurements, respectively. A general theoretical model is developed on how forward scattered light complicates the general applicability of Beer's law to the experimental UV-vis spectrum of scattering samples. The correlation between the scattering intensity and PSNP concentration is highly complicated with no robust linearity even when the scatterers' concentration is very low. Such complexity arises from the combination of concentration-dependence of light scattering depolarization and the scattering inner filter effects (IFEs). Scattering depolarization increases with the PSNP scattering extinction (thereby, its concentration) but can never reach unity (isotropic) due to the polarization dependence of the scattering IFE. The insights from this study are important for understanding the strengths and limitations of various scattering-based techniques for material characterization including nanoparticle quantification. They are also foundational for quantitative mechanistic understanding on the effects of light scattering on sample absorption and fluorescence measurements.

Integrating spheres (IS) have been used extensively for the characterization of light absorption in turbid samples. However, converting the IS-based sample absorption coefficient to the UV-vis absorbance quantified with a double-beam UV-vis spectrophotometer is challenging. Herein, we report an integrating-sphere-assisted resonance synchronous (ISARS) spectroscopy method performed with conventional spectrofluorometers equipped with an integrating-sphere accessory. Mathematical models and experimental procedures for quantifying the sample, solvent, and instrument-baseline ISARS intensity spectra were provided. A three-parameter analytical model has been developed for correlating the ISARS-based UV-vis absorbance and the absorbance measured with double-beam instruments. This ISARS method enables the quantitative separation of light absorption and scattering contribution to the sample UV-vis extinction spectrum measured with double-beam UV-vis spectrophotometers. Example applications of this ISARS technique are demonstrated with a series of representative samples differing significantly in their optical complexities, from approximately pure absorbers, pure scatterers, to simultaneous light absorbers, scatterers, and emitters under resonance excitation and detection conditions.

Polydopamine (PDA) is a unique bioinspired synthetic polymer that integrates broadband light absorption, efficient photothermal transduction, and versatile surface-adhesion functions in a single material entity. Here, we utilize colloidal PDA beads in the submicron particle size regime as an easily processable and photothermally active support for sub-10 nm Pd nanocatalysts to construct a multifunctional material system that allows us to kinetically boost thermal catalytic reactions through visible and near-infrared light illuminations. Choosing the Pd-catalyzed nitrophenol reduction by ammonium formate as a model transfer hydrogenation reaction exhibiting temperature-dependent reaction rates, we demonstrate that interfacial molecule-transforming processes on metal nanocatalyst surfaces can be kinetically modulated by harnessing the thermal energy produced through photothermal transduction in the PDA supports.

Reliable quantification of the optical properties of perovskite nanocrystals (PNCs) is necessary for rational PNC design and applications in optoelectronics. Presented herein is the experimental ev...

Dithiosulfindene (DTS) is a model molecule for dithiolethiones, a class of biomedically important sulfur-rich compounds with a general formula R1-S-S-C(=S)-R2. Reported herein is a combined experimental and computational study of DTS interactions with plasmonic gold nanoparticles (AuNPs) in water. DTS adsorptions and reactions on AuNPs were investigated using a combination of UV-vis, Raman, and surface enhanced Raman spectroscopic (SERS) methods. The saturation packing density of DTS on AuNPs is 596 pmol/cm2, and its structure and conformation on AuNP surfaces depends critically on its packing density. NaOH reacts with DTS both in water and on AuNPs, converting DTS into thiosalicyclic acid (TSA). TSA produced by NaOH reaction with DTS preadsorbed onto AuNPs partitions between remaining on AuNP surfaces and being released to the supernatant of the reaction solution. UV-vis measurements revealed that NaOH reaction with DTS in solution and on AuNPs both follow first order kinetics with a rate constant of 0.7...

Light scattering and absorption both contribute to the UV-vis extinction quantified with a UV-vis spectrophotometer, but they differ drastically in their causes and effects. The existing optical studies have generally focused on the fluorescence and total photon extinction of the aggregation-induced-emission (AIE) materials. The effects of aggregation on the light absorption and scattering of AIE luminogens (AIEgens) are essentially unexplored. Herein, we reported a spectroscopic study of the structures and properties of AIE aggregates using the combination of UV-vis, fluorescence, and polarized resonance synchronous spectroscopy (PRS2) methods. Optical activities including the light absorption extinction, scattering extinction, and absorbance-normalized fluorescence emission have been quantified for the first time for the AIE aggregates prepared with an in-house prepared AIEgen 2-((4-(diphenylamino)biphenyl-4-yl)methylene)malononitrile (TPMN). Absorption extinction dominates the entire UV-vis extinction spectrum obtained with the dissolved TPMN. For the UV-vis spectrum acquired with TPMN aggregates, however, scattering extinction accounts for up to 35% of the peak photon extinction at 480 nm. This number rises to 100% in the wavelength region beyond 580 nm. Sample incubation increases the light scattering extinction, scattering depolarization, and fluorescence emission of the TPMN aggregates, but reduces their light absorption extinction. With its ability to provide information inaccessible with the existing techniques, this work demonstrates that PRS2 is a powerful tool for AIE materials in quantifying their optical properties and monitoring their aggregation/disaggregation processes.

Kinetic chromogenic (CG) and fluorogenic (FG) quantification deduces analyte concentration based on the reaction rate between the CG/FG probe and its targeted molecule. Little progress has been made in the past half century in either the theory or the applications of the kinetic spectroscopic quantification methods. Current kinetic CG/FG quantification is limited only to a subset of CG/FG reactions that can be approximated as the single-step process, and more problematically, to research samples with no matrix interferences. Reported herein is a kinetic quantification model established for multistep CG/FG reactions and a proof-of-concept demonstration of direct kinetic FG quantification of biomarkers in practical samples. The kinetic spectral intensity of the CG/FG reactions with two rate-limiting steps comprises three temporal regions: an accelerating period where rate of signal change is increasingly rapid, a linear region where the rate of signal change is approximately constant, and a deceleration region where the rate of signal increase becomes progressively small. Kinetic quantification is performed through simple linear-curve-fitting of the kinetic signal in its linear time-course region. The theoretical model is validated with the dual CG/FG 2-thiobarbituric acid (TBA) and malondialdehyde (MDA) reaction. Proof-of-concept kinetic spectroscopic quantification of analytes in practical samples is demonstrated with the FG quantification of MDA in canned chicken. The only sample preparation is bench-top centrifugation followed by two sequential syringe filtrations. The total kinetic FG assay time is less than 10 min, more than 10 times more efficient than the current equilibrium-based MDA assay. The theoretical model and the measurement design strategies offered by this work should help transform the current kinetic spectroscopic quantification from a niche research tool to an indispensable technique for time-sensitive applications.

Reliable quantification of the optical properties of fluorescent quantum dots (QDs) is critical for their photochemical, -physical, and -biological applications. Presented herein is the experimental quantification of photon scattering, absorption, and on-resonance-fluorescence (ORF) activities of CdSe/CdS core/shell fluorescent QDs as a function of the shell sizes and geometries. Four spherical QDs (SQDs) with different diameters and four rod-like QDs (RQDs) with different aspect ratios (ARs) have been analyzed using UV-vis, fluorescence, and the recent polarized resonance synchronous spectroscopic (PRS2) methods. All quantum dots are simultaneous absorbers and scatterers in the UV-vis wavelength region, and they all exhibit strong ORF emission in the wavelength regions where the QDs both absorb and emit. The absorption and scattering cross-sections of the CdS shell are linearly and quadratically, respectively, proportional to the shell volume for both the SQDs and RQDs. However, the effects of CdS shell coating on the core optical properties are different between SQDs and RQDs. For RQDs, increasing the CdS shell volume through the length elongation has no effect on either the peak wavelength or intensity of the CdSe core UV-vis absorption and ORF, but it reduces the QD fluorescence depolarization. In contrast, increasing CdS shell volume in the SQDs induces red-shift in the CdSe core peak UV-vis absorption and ORF wavelengths, and increases their peak cross-sections, but it has no effect on the SQD fluorescence depolarization. The RQD ORF cross-sections and quantum yields are significantly higher than their respective counterparts for the SQDs with similar particle sizes (volumes). While these new insights should be significant for the QD design, characterization, and applications, the methodology presented in this work is directly applicable for quantifying the optical activities of optically complex materials where the common UV-vis spectrometry and fluorescence spectroscopy are inadequate.

C-Unsubstituted 1,2-diazetidines, a rarely studied type of four-membered heterocyclic compounds, were synthesized through an operationally simple intermolecular vicinal disubstitution reaction. 1,2-Diazetidine derivatives bearing various N-arylsulfonyl groups were readily accessed and studied by experimental and computed Raman spectra. The ring-opening reaction of the diazetidine was explored and resulted in the identification of a selective N-N bond cleavage with thiols as nucleophiles, which stereoselectively produced a new class of N-sulfenylimine derivatives with C-aminomethyl groups.

Ligand displacement from gold is important for a series of gold nanoparticle (AuNP) applications. Complete nondestructive removal of organothiols from aggregated AuNPs is challenging due to the strong Au–S binding, the steric hindrance imposed by ligand overlayer on AuNPs, and the narrow junctions between the neighboring AuNPs. Presented herein is finding that monohydrogen sulfide (HS–), an anionic thiol, induces complete and nondestructive removal of ligands from aggregated AuNPs. The model ligands include aliphatic (ethanethiol(ET)) and aromatic monothiols, methylbenzenethiol (MBT), organodithiol (benzenedithiol (BDT)), thioamides (mercaptobenzimidazole (MBI) and thioguanine (TG)), and nonspecific ligand adenine. The threshold HS– concentration to induce complete ligand displacement varies from 105 μM for MBI and TG to 60 mM for BDT. Unlike using HS–, complete ligand displacement does not occur when mercaptoethanol, the smallest water-soluble organothiol, is used as the incoming ligand. Mechanistically,...

Using the recent polarized resonance synchronous spectroscopic (PRS2) technique, we reported the quantification of photon extinction, absorption, scattering cross-section spectra, and scattering depolarization spectra for AuNPs of different sizes and shapes. The effects of the solvent composition, ligand functionalization, and nanoparticle aggregation on the AuNP photon absorption and scattering have also been experimentally quantified. The light scattering depolarization is close to 0 for gold nanospheres (AuNSs) crossing the entire UV-vis region but is strongly wavelength-dependent for gold nanorods (AuNRs). Increasing the dielectric constant of the medium surrounding AuNPs either by solvents or ligand adsorption increases photon absorption and scattering but has no significant impact on the AuNP scattering depolarization. Nanoparticle aggregation increases AuNP photon scattering. However, even the extensively aggregated AuNPs remain predominantly photon absorbers with photon scattering-to-extinction ratios all less than 0.03 for the investigated AuNP aggregates at the AuNP peak extinction wavelength. The AuNP scattering depolarization initially increases with the AuNP aggregation but decreases when aggregation further progresses. The insights from this study are important for a wide range of AuNP applications that involve photon/matter interactions, while the provided methodology is directly applicable for experimental quantification of optical properties for nanomaterials that are commonly simultaneously photon absorbers and scatterers.

Rayleigh scattering is a universal material property because all materials have nonzero polarizability. Reliable quantification of the material light scattering cross section in the liquid phase and its depolarization spectra is, however, challenging due to a host of sample and instrument issues. Using the recently developed polarized resonance synchronous spectroscopic method, we reported the light scattering cross section and depolarization spectra measured for a total of 29 liquids including water, methanol, ethanol, 1-propanol, 1-butanol, dimethylformamide, carbon disulfide, dimethyl sulfoxide, hexane and two hexane isomers (3-methylpentane and 2,3-dimethylbutane), tetrahydrofuran, cyclohexane, acetonitrile, pyridine, chloromethanes including di-, tri, tetrachloromethane, acetone, benzene and eight benzene derivatives (toluene, fluorobenzene, 1,2-, 1,3-, and 1,4-difluorobenzene, chlorobenzene, 1,2- and 1,3-dichlorobenzene, and nitrobenzene). The solvent light scattering depolarization is wavelength-independent for the model solvents, and it varies from 0.023 ± 0.011 for CCl

Proposed mechanisms of monolayer silver formation on gold nanoparticle (AuNP) include AuNP-facilitated under-potential reduction and antigalvanic reduction in which the gold reduces Ag+ into metallic atoms Ag(0). Reported herein is the spontaneous reactive Ag+ adsorption onto gold substrates that include both as-obtained and butanethiol-functionalized citrate- and NaBH4-reduced gold nanoparticles (AuNPs), commercial high-purity gold foil, and gold film sputter-coated onto silicon. The silver adsorption invariably leads to proton releasing to the solution. The nominal saturation packing density of silver on AuNPs varies from 2.8 ± 0.3 nmol/cm2 for the AuNPs preaggregated with KNO3 to 4.3 ± 0.2 nmol/cm2 for the AuNPs prefunctionalized with butanethiol (BuT). The apparent Langmuir binding constant of the Ag+ with the preaggregated AuNPs and BuT-functionalized AuNPs are 4.0 × 103 M–1 and 2.1 × 105 M–1, respectively. The silver adsorption has drastic effects on the structure, conformation, and stability of the...

Fluorophores are important but optically complicated photonic materials as they are simultaneous photon absorbers, emitters, and scatterers. Existing studies on fluorophore optical properties have been focused almost exclusively on its photon absorption and Stokes-shifted fluorescence (SSF) with scant information on the fluorophore photon scattering and on-resonance fluorescence (ORF). Presented herein is a unified theoretical framework and experimental approach for quantification of the fluorophore SSF, ORF, and scattering depolarization and anisotropy using a combination of fluorophore UV-vis, fluorescence emission, and resonance synchronous spectroscopic spectral measurements. A mathematical model for calculating fluorophore ORF and scattering cross sections has been developed that uses polystyrene nanoparticles as the external reference. The fluorophore scattering cross section is ∼10-fold smaller than its ORF counterparts for all the six model fluorophores, but more than 6 orders of magnitude larger than the water scattering cross section. Another finding is that the fluorophore ORF has a depolarization close to 1, while its Rayleigh scattering has zero depolarization. This enables the experimental separation of the fluorophore ORF and photon scattering features in the fluorophore resonance synchronous spectra. In addition to opening a new avenue for material characterization, the methods and insights derived from this study should be important for developing new analytical methods that exploit the fluorophore ORF and photon scattering properties.

Optical properties of fluorescent materials including their UV-vis absorption, scattering, and on-resonance fluorescence activities are strongly wavelength-dependent. Reported herein is a divide-and-conquer strategy for experimental quantification of fundamental optical constants of fluorescent nanomaterials including their UV-vis absorption, scattering, and on-resonance-fluorescence (ORF) cross-section spectra and ORF fluorescence and light scattering depolarization spectra. The fluorophore UV-vis extinction spectrum is first divided into a blue and a red wavelength region. The UV-vis extinction cross-section spectrum in the blue wavelength region is decomposed into its absorption and scattering extinction spectra straightforwardly using the established polarized resonance synchronous spectroscopic technique. In its red wavelength region, however, the fluorophores can be simultaneous photon absorbers, scatterers, and anti-Stokes-shifted, on-resonance, and Stokes-shifted fluorescence emitters under the resonance excitation and detection conditions. A polarized anti-Stokes'-shifted, on-resonance, and Stokes'-shifted spectroscopic method is developed for quantifying fluorophore absorption, scattering, one-resonance fluorescence (ORF) cross-section spectra, and scattering and ORF fluorescence depolarization spectra in this wavelength region. Example applications of the presented techniques were demonstrated with fluorescent polystyrene nanoparticles, fluorescent quantum dots, and molecular fluorophores Rhodamine 6G and Eosin Y.

Reported is a confined lattice plane electrochemical exfoliation method that exploits the electrochemical reaction of face (basal) or side (edge) planes of highly oriented pyrolytic graphite (HOPG) while other planes are blocked using wax, based on the anisotropy of HOPG for efficient and effective fabrication of graphene nanodots with uniform size distribution.

A series of recent works have demonstrated the spontaneous Ag+ adsorption onto gold surfaces. However, a mechanistic understanding of the Ag+ interactions with gold has been controversial. Reported herein is a systematic study of the Ag+ binding to AuNPs using several in-situ and ex-situ measurement techniques. The time-resolved UV-vis measurements of the AuNP surface plasmonic resonance revealed that the silver adsorption proceeds through two parallel pseudo-first order processes with a time constant of 16(±2) and 1,000(±35) s, respectively. About 95% of the Ag+ adsorption proceeds through the fast adsorption process. The in-situ zeta potential data indicated that this fast Ag+ adsorption is driven primarily by the long-range electrostatic forces that lead to AuNP charge neutralization, while the time-dependent pH data shows that the slow Ag+ binding process involves proton-releasing reactions that must be driven by near-range interactions. These experimental data, together with the ex-situ XPS measurement indicates that adsorbed silver remains cationic, but not as a charged-neutral silver atom proposed by the anti-galvanic reaction mechanism. The surface-enhanced Raman activities of the Ag+-stained AuNPs are slightly higher than that for AuNPs, but significantly lower than that for the silver nanoparticles (AgNPs). The SERS feature of the ligands on the Ag+-stained AuNPs can differ from that on both AuNPs and AgNPs. Besides the new insights to formation mechanism, properties, and applications of the Ag+-stained AuNPs, the experimental methodology presented in this work can also be important for studying nanoparticle interfacial interactions.

Electrolyte interactions with nanoparticles (NPs) at the solid/liquid interfaces are highly complicated as the charged species can be directly adsorbed onto the NP surfaces, confined in the diffusion layer immediately surrounding the NPs, and dispersed in bulk solutions. Existing studies on electrolyte interactions with NPs are based primarily on the electrical double layer theory that focuses mainly on electrolyte interactions with NPs with fixed pre-existing charges. Demonstrated herein is a comprehensive study of counterion effects during the electrolyte bindings to gold nanoparticles (AuNPs), including halide-induced AuNP aggregation and fusion, quantitative cation and anion coadsorption, selective cation and anion displacement on AuNPs, and surface-enhanced Raman spectroscopic features of the ionic species adsorbed onto AuNP surfaces. In contradiction to previous reports that electrolyte effects are anion-specific, we demonstrated that cations can play a dominant role in the halide-induced AuNP aggre...

Existing studies on molecular fluorescence have almost exclusively been focused on Stokes-shifted fluorescence spectroscopy (SSF) in which the emitted photon is detected at the wavelengths longer than that for the excitation photons. Information on fluorophore on-resonance fluorescence (ORF) and resonance Rayleigh scattering (RRS) is limited and often problematic due to the complex interplay of the fluorophore photon absorption, ORF emission, RRS, and solvent Rayleigh scattering. Reported herein is a relatively large-scale systematic study on fluorophore ORF and RRS using the conventional UV-vis extinction and SSF measurements in combination with the recently reported ratiometric resonance synchronous spectroscopic (R2S2, pronounced as "R-Two-S-Two") method. A series of fundamental parameters including fluorophore ORF cross sections and quantum yields have been quantified for the first time for a total of 12 molecular and 6 semiconductor quantum dot (QD) fluorophores. All fluorophore spectra comprise a well-defined Gaussian peak with a full width at half-maximum ranging from 4 to 30 nm. However, the RRS features of fluorophores differ drastically. The effect of fluorophore aggregation on its RRS, UV-vis, R2S2, and SSF spectra was also discussed. This work highlights the critical importance of the combined UV-vis extinction, SSF, and R2S2 spectroscopic measurements for material characterizations. The method and insights described in this work can be directly used for improving the reliability of RRS spectroscopic methods in chemical analysis. In addition, it should pave the way for developing novel R2S2-based analytical applications.

Organothiol binding to gold nanoparticles (AuNPs) in water proceeds through a deprotonation pathway in which the sulfur-bound hydrogen (RS-H) atoms are released to solution as protons and the organothiol attach to AuNPs as negatively charged thiolate. The missing puzzle pieces in this mechanism are (i) the significance of electrostatic repulsion among the likely charged thiolates packed on AuNP surfaces, and (ii) the pathways for the ligand binding system to cope with such electrostatic repulsion. Presented herein are a series of experimental and theoretical evidence that ion pairing, the coadsorption of negatively charged thiolate and positively charged cations, is a main mechanism for the system to reduce the electrostatic repulsion among the thiolate self-assembled onto AuNP surfaces. This work represents a significant step forward in the comprehensive understanding of organothiol binding to AuNPs.

Multicomponent ligand interactions are involved in essentially all practical nanoparticle (NP) applications. Presented herein is the finding that multicomponent ligand binding to gold nanoparticles (AuNPs) can be highly dependent on the sequence of ligand mixing with AuNPs. Quantitative study revealed that the competitive adenine and glutathione (GSH) adsorption onto both as-synthesized and pegylated AuNPs is predominantly kinetically controlled, and adenine that binds only nonspecifically to AuNP adsorbs faster than GSH. This raises concerns about the validity of the popular practice in current NP research of using the Langmuir isotherm or its variants to model multicomponent ligand adsorption on NPs. Mechanistically, this sequence dependency is due to the fact that there is no spontaneous ligand desorption even for the model protein and small molecules that can bind only nonspecifically to AuNPs. The insights and experimental methods provided in this work should be important for molecular-level understa...

Anisotropy and depolarization are two interconvertible parameters in fluorescence and light scattering spectroscopy that describe the polarization distribution of emitted and scattered photons generated with linearly polarized excitation light. Whereas anisotropy is more frequently used in fluorescence literature for studying association/dissociation of fluorophore-bearing reagents, depolarization is more popular in the light-scattering literature for investigating the effect of scatterers' geometries and chemical compositions. Presented herein is a combined computational and experimental study of the scattering and fluorescence depolarization enhancement induced by light scattering in turbid samples. The most important finding is that sample light scattering and fluorescence depolarization increases linearly with sample light-scattering extinction. Therefore, one can extrapolate the analyte-specific scattering and fluorescence depolarization through linear curve fitting of the sample light scattering and fluorescence depolarization as a function of the sample concentration or the path length of the sampling cuvettes. An example application of this linear extrapolation method is demonstrated for quantifying the fluorophore-specific fluorescence depolarization and consequently its anisotropy for an aggregation-induced-emission sample. This work should be important for a wide range of macromolecular, supramolecular, and nanoscale fluorescent materials that are often strong light scatterers due to their large sizes.

A series of recent works have demonstrated the spontaneous Ag+ adsorption onto gold surfaces. However, a mechanistic understanding of the Ag+ interactions with gold has been controversial. Reported herein is a systematic study of the Ag+ binding to AuNPs using several in-situ and ex-situ measurement techniques. The time-resolved UV-vis measurements of the AuNP surface plasmonic resonance revealed that the silver adsorption proceeds through two parallel pseudo-first order processes with a time constant of 16(±2) and 1,000(±35) s, respectively. About 95% of the Ag+ adsorption proceeds through the fast adsorption process. The in-situ zeta potential data indicated that this fast Ag+ adsorption is driven primarily by the long-range electrostatic forces that lead to AuNP charge neutralization, while the time-dependent pH data shows that the slow Ag+ binding process involves proton-releasing reactions that must be driven by near-range interactions. These experimental data, together with the ex-situ XPS measurement indicates that adsorbed silver remains cationic, but not as a charged-neutral silver atom proposed by the anti-galvanic reaction mechanism. The surface-enhanced Raman activities of the Ag+-stained AuNPs are slightly higher than that for AuNPs, but significantly lower than that for the silver nanoparticles (AgNPs). The SERS feature of the ligands on the Ag+-stained AuNPs can differ from that on both AuNPs and AgNPs. Besides the new insights to formation mechanism, properties, and applications of the Ag+-stained AuNPs, the experimental methodology presented in this work can also be important for studying nanoparticle interfacial interactions.

With their unique photochemical properties, porphyrins have remained for decades the most interested chemicals as photonic materials for applications ranging from chemistry, biology, medicine, to photovoltaic. Porphyrins can self-assemble into higher order structures. However, information has been scant on the kinetics and structural evolution during porphyrin assembly and disassembly. Furthermore, quantitative understanding of the porphyrin optical activities is complicated by the complex interplay of photon absorption, scattering, and fluorescence emission that can concurrently occur in porphyrin samples. Using meso-tetrakis(4-sulfonatophenyl)porphyrin as the model molecule, reported herein is a combined UV-vis extinction, polarized Stokes-shifted fluorescence, and polarized resonance synchronous spectroscopic (PRS2) study of porphyrin assembly and disassembly in acidic solutions. Although porphyrin assembly and disassembly occur instantaneously upon the sample preparation, both processes last at least a few months before reaching their approximate equilibrium states. The two processes were monitored in situ by quantifying the porphyrin fluorescence and scattering depolarizations as well as its extinction, absorption, scattering, and fluorescence emission cross sections. In addition to a series of new insights to the porphyrin assembly and disassembly, the methodology described in this work opens the door for the in situ study of the structural and optical properties of photonic materials comprising molecular assembly.

The sample inner filter effect (IFE) induces spectral distortion and affects the linearity between intensity and analyte concentration in fluorescence, Raman, surface enhanced Raman, and Rayleigh light scattering measurements. Existing spectrofluorometric-based measurements treat light scattering and absorption identically in their sample IFEs. Reported herein is the finding that photon scattering and absorption differ drastically in inducing the sample IFE in Stokes-shifted fluorescence (SSF) spectrum, resonance synchronous spectrum (RS2), and the polarized resonance synchronous spectrum (PRS2) measurements. Absorption with an absorption extinction as small as 0.05 imposes significant IFE on SSF, RS2, and PRS2 measurements. However, no significant IFE occurs even when the scattering extinction is as high as 0.9. For samples that both absorb and scatter light, one should decompose their UV-vis extinction spectra into absorption and scattering extinction component spectra before correcting the sample IFE. An iteration PRS2 method was introduced for the experimental decoupling of the photon absorption and scattering contribution. The methodology presented in this work can be easily implemented by researchers with access to one conventional UV-vis spectrophotometer and one spectrofluorometer equipped with a pair of excitation and detection polarizers. This work should be of broad significance in chemical research given the popularity of fluorescence spectroscopy in material characterization applications.

Presented herein is the ratiometric bandwidth-varied polarized resonance synchronous spectroscopy (BVPRS2) method for quantification of material optical activity spectra. These include the sample light absorption and scattering cross-section spectrum, the scattering depolarization spectrum, and the fluorescence emission cross-section and depolarization spectrum in the wavelength region where the sample both absorbs and emits. This ratiometric BVPRS2 spectroscopic method is a self-contained technique capable of quantitatively decoupling material fluorescence and light scattering signal contribution to its ratiometric BVPRS2 spectra through the linear curve-fitting of the ratiometric BVPRS2 signal as a function of the wavelength bandwidth used in the PRS2 measurements. Example applications of this new spectroscopic method are demonstrated with materials that can be approximated as pure scatterers, simultaneous photon absorbers/emitters, simultaneous photon absorbers/scatterers, and finally simultaneous photon absorbers/scatterers/emitters. Because the only instruments needed for this ratiometric BVPRS2 technique are the conventional UV-vis spectrophotometer and spectrofluorometer, this work should open doors for routine decomposition of material UV-vis extinction spectrum into its absorption and scattering component spectra. The methodology and insights provided in this work should be of broad significance to all chemical research that involves photon/matter interactions.

Three gold-nanoparticle (AuNP) undergraduate experiment modules that are focused on nanoparticles interfacial phenomena have been developed. Modules 1 and 2 explore the synthesis and characterization of AuNPs of different sizes but with the same total gold mass. These experiments enable students to determine how particle size affects the AuNP optical properties and ligand binding capacities. Module 3 investigates the fundamental mechanism governing organothiol self-assembly onto AuNPs and explores the fate of the sulfur-bounded hydrogen (RS–H) for organothiols on the AuNP surface. A benchtop centrifuge, a UV–vis spectrophotometer, and pH strips are needed. The depth of required chemistry knowledge is appropriate for upper-level chemistry students.

Crystallographic structures of carbon-encapsulated iron oxide nanoparticles, derived from the carbonization of cellulose were investigated. The phase transitions of carbon-encapsulated iron oxide nanoparticles were characterized and analyzed after pyrolysis treatment at four temperatures: 500 °C, 800 °C, 1000 °C and 1600 °C, respectively. Transmission Electronic Microscope (TEM) and X-ray diffraction (XRD) techniques were employed for the analysis. Samples treated at 500 °C spontaneously combusted immediately after being exposed to the air indicating the existence of α-Fe2O3 particles. Core-shell structures consisting of dark grains and a light matrix with graphitic structure appeared in samples treated at 800 °C. XRD and TEM selected area electron diffraction demonstrated that between 800 °C and 1600 °C, catalytic graphitization occurred, and the iron carbide/iron crystalline phase was developed for the cores. It was found that the phase transition from Fe3C to Fe occurred at 1600 °C.

Using propanethiol (PrT), 2-mercaptoethanol (ME), glutathione (GSH), and cysteine (Cys) as model thiols, we demonstrated herein that organothiols can induce both silver nanoparticle (AgNP) disintegration and formation under ambient conditions by simply mixing organothiols with AgNPs and AgNO3, respectively. Mechanistically, organothiols induce AgNP disintegration by chelating silver ions produced by ambient oxygen oxidizing the AgNPs, while AgNP formation in AgNO3/organothiol mixtures is the result of organothiols serving as the reducing agent. Furthermore, surface-plasmon- and fluorescent-active AgNPs can be interconverted by adding excess Ag+ or ME into the AgNP-containing solutions. Organothiols can also reduce gold ion in HAuCl4/organothiol solutions into fluorescence- and surface-plasmon-active gold nanoparticles (AuNPs), but no AuNP disintegration occurs in the AuNP/organothiol solutions. This work highlights the extraordinary complexity of organothiol interactions with gold and silver nanoparticles...

Ion-pairing, the association of oppositely charged ionic species in solution and at liquid/solid interfaces has been proposed as a key factor for a wide range of physicochemical phenomena. However, experimental observations of ion pairing at the ligand/solid interfaces are challenging due to difficulties in differentiating ion species in the electrical double layer from that adsorbed on the solid surfaces. Using surface enhanced Raman spectroscopy in combination with electrolyte washing, we presented herein the first direct experimental evidence of ion pairing, the coadsorption of oppositely charged ionic species onto gold nanoparticles (AuNPs). Ion pairing reduces the electrolyte concentration threshold in inducing AuNP aggregation and enhances the competitiveness of electrolyte over neutral molecules for binding to AuNP surfaces. The methodology and insights provided in this work should be important for understanding electrolyte interfacial interactions with nanoparticles.

Organodithiols with two distal thiols have been used extensively in gold and silver nanoparticle (AuNP and AgNP) applications. However, understanding the structures and conformations of organodithiols on these nanoparticles is challenging. Reported in this work is a combined surface enhanced Raman spectroscopy (SERS), transmission electron microscope (TEM), inductively coupled plasma mass-spectrometry (ICP-MS), and localized surface plasmonic resonance (LSPR) study of alkyldithiol (ADT, (HS-(CH2)n-SH, n = 2, 4, and 6) interactions with AuNPs and AgNPs in water. These complementary techniques revealed a series of new insights that would not be possible using individual methods. A large-fraction of ADTs lies flat on AuNP surfaces. The upright ADTs are dimerized horizontally through disulfide-bond, or remain as monothiolates on the AuNP surfaces. The possibility of a significant amount of vertically disulfide-linked organodithiol on the surface is excluded on the basis of ICP-MS and AuNP LSPR experiments. AD...

Para-aryl-dithiols (PADTs, HS-(C6H4)n-SH, n = 1, 2, and 3) have been used extensively in molecular electronics, surface-enhanced Raman spectroscopy (SERS), and quantum electron tunneling between two gold or silver nanoparticles (AuNPs and AgNPs). One popular belief is that these dithiols cross-link noble metal nanoparticles (NPs) as monolayer dithiolate spacers. Reported herein is our finding that PADTs predominantly exist as monothiolate forms on AuNPs or AgNPs. No PADT-induced NP cross-linking was observed regardless of the NP/PADT concentration ratios. Moreover, only one PADT thiol can be deprotonated even when PADTs are treated with concentrated NaOH or AgNO3 solutions. In contrast, 1,4-benzenedimethanethiol (HS-CH2-(C6H4)1-CH2-SH) and alkyl dithiol 1,2-ethanedithiol can be dithiolated on AuNPs and AgNPs, and in excess NaOH and AgNO3 solutions. This study should be of broad importance for plasmonic NP research given the popularity of PADTs in molecular electronics and SERS applications.

Gold nanoparticles (AuNPs) have been used extensively as surface-enhanced Raman spectroscopic (SERS) substrates for their large SERS enhancements and widely believed chemical stability. Presented is the finding that iodide can rapidly reduce the SERS intensity of the ligands, including organothiols adsorbed on plasmonic AuNPs through both iodide-induced ligand desorption and AuNP fusion. The organothiols trapped inside the fused AuNPs have negligible SERS activities. Multiple photochemical processes were involved when organothiol-containing AuNP aggregates were treated with KI under photoillumination. The photocatalytically produced I3– reacts with both organothiol and AuNPs. Chloride and bromide also induce partial organothiol displacement and the fusion of the as-synthesized AuNPs, but neither of the two halides has detectable effects on the morphology and Raman signals of the organothiol-containing AuNP aggregates. The insight provided in this work should be important for the understanding of interfaci...

The stability of citrate-residues on gold nanoparticles (AuNPs) against ligand displacement has been controversial. Using AuNPs synthesized with deuterated citrate in combination with in-situ surface-enhanced Raman spectroscopic (SERS) analysis, we report that both citrate-residues and solution impurities can be simultaneously adsorbed onto citrate-reduced AuNPs in solution. The citrate-residues can be readily displaced from AuNPs by organosulfur such as organothiols (RS-H), organodisuflide (R-S-S-R), and non-specific ligands including halides and adenine. Control experiments conducted on high-purity gold films sputter-coated onto silicone substrates indicate that air-borne and solvent-borne impurities rapidly contaminate the gold surfaces. Head-to-head comparison of ligand-functionalized AuNPs by in-situ SERS measurements verses those from the ex-situ X-ray photoelectron spectroscopic (XPS) measurements reveal that the impurity deposition can compromise the reliability of ex-situ XPS identification of surface adsorbates on AuNPs in solution. These insights are of general significance to nanoscience research given the broad interest in nanoparticle surface chemistry and popularity of XPS for nanomaterial characterizations.

Dispersion stability, ligand structure and conformation, and SERS activities of 1-alkanethiol (CnH2n+1SH, n = 2–14) functionalized gold and silver nanoparticles (AuNPs and AgNPs) were studied as a function of alkanethiol carbon chain length and nanoparticle (NP) type and size. The dispersion stability of alkanethiol functionalized NPs in water increases with increasing alkanethiol chain length and NP size, and the stabilities of the alkanethiol-containing AuNPs are higher than their AgNP counterparts. C3H7SH and longer alkanethiols are highly ordered on AgNPs but disordered on AuNPs. The SERS intensity of the C–S stretch band for the model alkanethiols on AgNPs and AuNPs decays exponentially (I = I0 exp(−Nc/N0)) with increasing number of carbon atoms (Nc). The empirical decay length N0, in terms of the number of the carbon atoms, is 1.29, 0.53, and 0.10 for AgNPs with diameters of 50, 30, and 10 nm, respectively. This decay length is less than 1 for AuNPs of difference sizes. These results show that chang...

Previous researchers proposed that thiolated poly(ethylene glycol) (PEG-SH) adopts a “mushroom-like” conformation on gold nanoparticles (AuNPs) in water. However, information regarding the size and permeability of the PEG-SH mushroom caps and surface area passivated by the PEG-SH mushroom stems are unavailable. Reported herein is our finding that AuNPs that are covered by saturation packed PEG-SHs all have large fractions of AuNP surface area available for ligand adsorption and exchange. The model ligands adenine and 2-mercaptobenzimidazole (2-MBI) can rapidly penetrate the PEG-SH overlayer and adsorb onto the AuNP surface. Most of the ligand adsorption and exchange occurs within the first minutes of the ligand addition. The fraction of AuNP surface area passivated by saturation packed model PEG-SHs are ∼25%, ∼20%, and ∼9% for PEG-SHs with molecular weights of 2000, 5000, and 30 000 g/mol, respectively. Localized surface plasmonic resonance and dynamic light scattering show that the PEG-SH overlayer is dr...

We recently reported that a wide range of ligands, including organothiols (OTs), can be completely desorbed from gold nanoparticles (AuNPs) by NaBH4 in water. In addition, NaBH4 induces desulfurization of 2-mercaptobenzimidazole (2-MBI) and 6-thioguanine (6-TG) on AuNPs. Reported herein is a systematic investigation of treating ligands adsorbed onto silver nanoparticles (AgNPs) with NaBH4. These results are compared and contrasted to those previously reported for the same set of ligands adsorbed onto AuNPs. Complete desorptions from AgNPs by NaBH4 in water were observed for nonspecifically adsorbed ligands that include Rhodamine 6G, adenine, thiophene, and halides (Cl–, Br–, and I–). These cleaned AgNPs can be reused for surface-enhanced Raman spectroscopy acquisition. However, OT ligands could not be completely desorbed from AgNPs regardless of the amount of NaBH4 used in this work. NaBH4 can induce complete 6-TG desulfurization adsorbed on AgNPs, but the desulfurization rate is significantly slower than...

Gold nanoparticles (AuNPs) are an attractive delivery vector in biomedicine because of their low toxicity and unique electronic and chemical properties. AuNP bioconjugates can be used in many applications, including nanomaterials, biosensing, and drug delivery. While the phenomenon of spontaneous protein–AuNP adsorption is well-known, the structural and mechanistic details of this interaction remain poorly understood. As a result, predicting the orientation and structure of proteins on the nanoparticle surface remains a challenge. New techniques are therefore needed to characterize the structural properties of proteins as they bind to AuNPs. We have developed a straightforward and rapid NMR-based approach to quantitatively characterize the protein–AuNP interaction. This approach is immune to the inner filter effect, which complicates fluorescence measurements, and it can be performed without prior centrifugation of samples. Using a data set of six proteins, ranging in size from 3 to 583 residues, we measu...

Organothiol (R-SH) (OT) adsorption onto silver nanoparticles (AgNPs) in water was studied for a series of aromatic OTs including p-methylbenzenethiol (p-MBT), p-benzenedithiol (p-BDT), and 2-mercaptobenzimidazole (2-MBI). Unlike the common view that OT forms monolayer adsorption on AgNPs, we found that these aromatic OTs continuously reacted with AgNPs through formation of RS–Ag complexes until complete OT or AgNP consumption occurred. The RS–Ag complex can remain on the AgNP surface, converting the AgNPs into core–shell structures. The OT adsorption onto AgNPs occurs predominately through reaction with silver oxide present on the AgNP surfaces before the OT addition or formed from environmental oxygen in the presence of OT. The RS–H protons are completely released when both p-MBT and 2-MBI reacted with AgNP, Ag2O, and AgNO3. However, a substantial fraction of S–H bonds remained intact when p-BDT, the only dithiol used in this work, is adsorbed on AgNPs or reacted independently with Ag2O and AgNO3. The ne...

The resonance Raman (RR) enhancement factors of Rhodamine 6G (R6G) in water and on gold and silver nanoparticles (AuNPs and AgNPs) were determined using a double ratiometric method where adenine is used as the internal reference. The RR enhancement factor for R6G on AgNPs upon laser excitation at 532 nm is 537.6 ± 214.8. This is ∼5 times lower than the experimental (2.7 ± 0.3) × 103 RR enhancement factor for R6G in water. These experimental RR enhancement factors for R6G in water and on AgNPs are 104 smaller than the 107 RR enhancement proposed in literature for R6G in water and on SERS substrates. In addition, a simple back-of-the-envelope calculation showed that even with this damped RR for R6G on AgNPs in comparison to R6G in water, a SERS enhancement factor of 106 is sufficient to explain the single-molecule resonance SERS activities reported for R6G located in nanoparticle junctions. This conclusion is deduced from fact that normal Raman spectrum could be readily obtained with 24 fmol of adenine at l...

The role of cysteine residues in the protein binding kinetics and stability on gold nanoparticles (AuNP) was studied using AuNP localized surface plasmon resonance (LSPR) in combination with an organothiol (OT) displacement method. GB3, the third IgG-binding domain of protein G, was used to model protein-AuNP adsorption. While wild-type GB3 (GB30) contains no cysteine residues, bioengineered GB3 variants containing one (GB31) and two (GB32) cysteine residues were also tested. The cysteine content has no significant effect on GB3 binding kinetics with AuNPs, and most protein adsorption occurs within the first few seconds upon protein/AuNP mixing. However, the stability of GB3 on the AuNP surface against OT displacement depends strongly on the cysteine content and the age of the AuNP/GB3 mixture. The GB30 covered AuNPs can be completely destabilized and aggregated by OTs, regardless of the age of the GB30/AuNP mixtures. Long-time incubation of GB31 or GB32 with AuNPs can stabilize AuNPs against the OT adsorption inducted aggregation. This study indicates that multiple forces involved in the GB3/AuNP interaction, and covalent binding between cysteine and AuNP is essential for a stable protein/AuNP complex.

Organosulfur compounds are known to poison metallic nanoparticle catalysts. Herein NaBH4 is shown to desorb and desulfurize 2-mercaptobenzimidazole (2-MBI) and 6-thioguanine (6-TG) adsorbed on 10, 15, and 50 nm diameter gold nanoparticles (AuNPs). The desulfurization rates decrease significantly with increasing AuNP sizes. Isotope labeling experiments, conducted with NaBD4 in H2O, indicate that this desulfurization reaction proceeds through a pathway requiring hydrogen uptake onto AuNP surfaces prior to the 2-MBI or 6-TG desulfurization reaction, rather than direct hydride attack from BH4– on the sulfur-bearing carbon in 2-MBI or 6-TG, or H2 reaction with 2-MBI or 6-TG . In addition to serving as the hub for electron charge transfer between hydride and proton, AuNPs capture the cleaved sulfide, facilitating sulfur separation from the desulfurized products.