Optical Absorption Cross Section Research Articles

Photon counting based pump-probe technique for quantitative characterization of fluorescence in a lock-in free detection manner

The stimulated emission (SE) signal in pump-probe experiment is conventionally measured with lock-in detection to differentiate the weak signals from the relatively large background of spontaneous emission and probe beam. Therefore, direct characterization of signal strength are often major limiting factors in terms of noise, speed, and data acquisition. In contrast, photon counting allows direct quantification of signal strength, while synchronized pump-probe pulse enables precise timing and the separation of signals accordingly. Herein, the SE based pump-probe method is combined with time-correlated single-photon counting to investigate the ultrafast photochemical parameters, digitally and quantitatively. As a proof-of-concept, our technique is applied to investigate, fluorescence lifetime (τ)∼3.71ns , optical absorption cross-section (σabs)∼1.23×10−16cm2 , and the SE cross-section (σSE)∼2.22×10−17cm2 , of a fluorescent dye (ATTO 647N) quantitatively. The experimental results are also compared with theoretical photon statistics to further justify the advantages including experimental and statistical critical molecular dynamics parameters extraction with excellent high accuracy.

A new instrumentation for simultaneous terahertz and mid-infrared spectroscopy in corrosive gaseous mixtures.

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200-350K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200-350K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42m and by a THz radiation beam (82.5GHz to 1.1THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.

Multidimensional Widefield Infrared-Encoded Spontaneous Emission Microscopy: Distinguishing Chromophores by Ultrashort Infrared Pulses.

Photoluminescence (PL) imaging has broad applications in visualizing biological activities, detecting chemical species, and characterizing materials. However, the chemical information encoded in the PL images is often limited by the overlapping emission spectra of chromophores. Here, we report a PL microscopy based on the nonlinear interactions between mid-infrared and visible excitations on matters, which we termed MultiDimensional Widefield Infrared-encoded Spontaneous Emission (MD-WISE) microscopy. MD-WISE microscopy can distinguish chromophores that possess nearly identical emission spectra via conditions in a multidimensional space formed by three independent variables: the temporal delay between the infrared and the visible pulses (t), the wavelength of visible pulses (λvis), and the frequencies of the infrared pulses (ωIR). This method is enabled by two mechanisms: (1) modulating the optical absorption cross sections of molecular dyes by exciting specific vibrational functional groups and (2) reducing the PL quantum yield of semiconductor nanocrystals, which was achieved through strong field ionization of excitons. Importantly, MD-WISE microscopy operates under widefield imaging conditions with a field of view of tens of microns, other than the confocal configuration adopted by most nonlinear optical microscopies, which require focusing the optical beams tightly. By demonstrating the capacity of registering multidimensional information into PL images, MD-WISE microscopy has the potential of expanding the number of species and processes that can be simultaneously tracked in high-speed widefield imaging applications.

Determining the Optical Geometry of a Gold Semi-Shell under the Kretschmann Configuration

Dielectric nanoparticles coated with metals (half-shell or semi-shell structures) have attracted attention as potential composite plasmonic nanomaterials with large optical anisotropy and absorption cross-sections. Structures approximately 100 nm in size can excite plasmons in the visible and near-infrared ranges, highlighting their distinct optical properties. This study employed metal semi-shell structures (metal: gold, dielectric: silica) in the Kretschmann configuration to experimentally and numerically demonstrate the optical determination of single-structure orientations through a finite-difference time-domain method. Gold semi-shell structures were fabricated through deposition and etching. These structures were removed from their substrate in ultrapure water and randomly dropped onto a thin gold substrate. In the single structure, we experimentally observed changes in the scattering light spectrum based on the optical geometry of the gold semi-shell at wavelengths ranging from 530 to 700 nm. The obtained results closely resembled those of a simulation and confirmed the presence of eigenmodes in the orientation through electric field analysis. These observations allow for the cost-effective and rapid determination of the orientations of numerous structures that are approximately 100 nm in size, solely through optical methods. This technique is a valuable development for measurement applications in nanostructure orientation control and functionality enhancement.

Effect of barrier layer width on the optical and spectral properties of InAsP/AlGaInP quantum dot lasers

The barrier layer thickness separating the dot-in-well structure in optoelectronic devices plays a crucial impact on the performance of these devices. Here, we report the effect of the barrier width in quantum dot (QD) laser. Three series of InAsP QD lasers were papered with different barrier widths (i.e., 8, 16, and 24 nm) of (AlGa)InP. The samples were prepared by MOVPE at 730 °C. Data of threshold current density, Jth giving a lower Jth for 8 nm barrier width sample and it becomes more temperature dependency evident by a low value of characteristic temperature, To ≈ 77 k. Emission spectra of 2 mm long lasers showed a blue-shift by approximately 11 nm when the barrier width increased from 8 to 24 nm. The modal absorption study showed a decrease in the inhomogeneous broadening for 24 nm barrier width sample. Simulation results revealed an increase in the optical absorption cross-section, σo with increasing barrier width. These results showed that significant improvements in the spectral properties of the InAsP QD lasers can be made by altering the (AlGa)InP barrier width.

Multichromatic quantum superpositions in entangled two-photon absorption spectroscopy

Quantum information science is driving progress in a vast number of scientific and technological areas that cover molecular spectroscopy and matter-light interactions in general. In these fields, the ability to generate quantum mechanically-entangled photons is opening avenues to explore the interaction of molecules with quantum light. This work considers an alternative way of correlating photons by including energy superpositions. We study how the multichromatic quantum superposition, or color superposition of photon-pair states, influences the optical properties of organic chromophores. This work uses electronic structure calculations based on time-dependent density functional theory, and a simple modification of the standard entangled two-photon absorption theory. Our calculations show that it is possible to substantially modify the optical absorption cross section of molecules, where constructive and destructive interferences are computed. The quantum interference effects are more pronounced than the constructive ones. These quantum effects, or related ones, could be observed in quantum spectroscopic experiments where qudit photon states are generated.

Zn_{1-x}Ni_xTe semiconductor nanocrystals in transparent glass for optoelectronic device applications

Doping glass with semiconductors, particularly with nanostructured semiconductors, has attracted attention due to the large optical absorption cross-sections of the latter. Based on this property, Ni^{2+} (5 wt%) doped phosphate glass and Zn_{1-x}Ni_xTe (x = 0.5, 1.0, 5.0 and 10.0 wt% of Ni^{2+}) nanocrystals (NCs) doped phosphate glasses (GCs) were prepared by fusion method and subsequent heat treatment. Influence of Ni^{2+} on structural, thermo-optical and third-order nonlinear optical properties have been analysed through various spectroscopic characterizations. The XRD pattern of the glass (G) exhibits the amorphous nature of the host material while GCs exhibit not only amorphous halo but also the presence of quantum dots (QDs) or nanocrystals (NCs) phases. TEM analysis of the studied GCs samples confirm the presence of quantum dots (QDs) and bulk NCs with an average diameter of approximately 4.2 {pm } 0.3 nm and 13.4 {pm } 0.2 nm, respectively. Several phosphate groups were observed and reported from Raman and FTIR-ATR spectra. The absorption band positions confirmed that Ni^{2+} ions resemble to the octahedral symmetry. The intensity of absorption band around 1352 nm (^3T{_1}(F) rightarrow^3A{_2}(F)) increased with the increase of Ni^{2+} in GCs which is an indicative of the ^{[6]}Ni^{2+} coordination. The emission properties such as emission cross-sections ({sigma }_{emi}) full width at half maxima (FWHM) for the ^1T{_2}(D) rightarrow^3T{_2}(F) (visible) and ^3A{_2}(F) rightarrow^3T{_1}(F) (near-infrared) emission transitions were reported. Among the glass-containing semiconductor nanocrystals (GCs), the emission cross-sections in GC4 sample (x = 10% of Ni^{2+}) are the largest for both the visible (11.88 times 10^{-18} cm^2) and infrared (0.98 times 10^{-20} cm^2) transitions. Thermal diffusivity (D), thermal conductivity (K) and temperature dependent optical path length change (ds/dT) were obtained through time-resolved thermal lens (TL) and thermal relaxation (TR) methods. The D and K parameters do not change significantly with increase of Ni^{2+} ions (0.5–5%) in GCs. Nonlinear-refractive index and nonlinear absorption of the studied samples were also obtained using femtosecond Z-scan technique. The increase of nonlinear absorption coefficient (beta) is observed from GC2 (2.53 {times } 10^{-10} cm/W) to GC4 (7.98 {times } 10^{-10} cm/W). The GC4, sample with 10 wt% of Ni^{2+}, showed the lowest ds/dT (1.22 times 10^{-6} K^{-1}) with good lasing (FOM and emission cross-sections) and nonlinear absorption properties suggesting that it can be a good candidate for visible-red emission light conversion in LED technology.

Photoinhibition of the Picophytoplankter Synechococcus Is Exacerbated by Ocean Acidification

The marine picocyanobacterium Synechococcus accounts for a major fraction of the primary production across the global oceans. However, knowledge of the responses of Synechococcus to changing pCO2 and light levels has been scarcely documented. Hence, we grew Synechococcus sp. CB0101 at two CO2 concentrations (ambient CO2 AC:410 μatm; high CO2 HC:1000 μatm) under various light levels between 25 and 800 μmol photons m−2 s−1 for 10–20 generations and found that the growth of Synechococcus strain CB0101 is strongly influenced by light intensity, peaking at 250 μmol m−2 s−1 and thereafter declined at higher light levels. Synechococcus cells showed a range of acclimation in their photophysiological characteristics, including changes in pigment content, optical absorption cross section, and light harvesting efficiency. Elevated pCO2 inhibited the growth of cells at light intensities close to or greater than saturation, with inhibition being greater under high light. Elevated pCO2 also reduced photosynthetic carbon fixation rates under high light but had smaller effects on the decrease in quantum yield and maximum relative electron transport rates observed under increasing light intensity. At the same time, the elevated pCO2 significantly decreased particulate organic carbon (POC) and particulate organic nitrogen (PON), particularly under low light. Ocean acidification, by increasing the inhibitory effects of high light, may affect the growth and competitiveness of Synechococcus in surface waters in the future scenario.

Sunlight propelled two-dimensional nanorobots with enhanced mechanical damage of bacterial membrane

Bacterial pollution in water sources poses a serious threat to human health and causes a water crisis. To treat it efficiently and ecologically, many studies have explored the antibacterial properties of two-dimensional nanomaterials in water, but their static antibacterial modes limit their effectiveness. In this work, we designed a facile and effective antibacterial nanorobots by loading super small gold nanorods (sAuNR) onto the surface of MXene nanosheets (MXene@sAuNR). The plasmon resonance effect of sAuNR can enhance the optical absorption cross section of the nanorobots, thereby improving their motion ability under irradiation and then causing cell membrane mechanical damage to bacteria. Our research proved that nanorobots with good optical driving characteristics displayed gratifying antibacterial properties even at ultra-low concentration as 5 µg/mL within 30 min. Furthermore, the nanorobots showed satisfactory antibacterial efficiency in real river samples under sunlight irradiation. These nanorobots presented in this study provides valuable insights towards designing self-energy collection and self-driving antibacterial materials that overcome the shortcomings of conventional static antibacterial methods. As sunlight is the cheapest and natural light source, these nanorobots have opened an effective and sustainable way for large-scale treatment of bacterial pollution in water.

Plasmonic Metasurface Integrated Black Phosphorus‐Based Mid‐Infrared Photodetector with High Responsivity and Speed

AbstractBlack Phosphorus (BP), a van der Waals (vdW) semiconducting material, is studied intensively due to its unique structural and anisotropic optical properties. BP exhibits optical anisotropy with thickness tunable direct bandgap from 0.3–2.0 eV. Many BP‐based photodetectors operating in the mid‐infrared (MIR) band are proposed, but their performance is still limited due to the small optical absorption cross section at room temperature. In this work, a plasmonic metasurface that has localized surface plasmon resonance (LSPR) at 3.7 µm is designed, matching well with the band edge of a BP flake and field confinement around the edge of Au‐disk as confirmed by finite difference time domain simulation and Fourier‐transform infrared spectroscopy measurement results. In addition, a BP flake is integrated on the plasmonic metasurface and a significant quenching (twelve‐fold) is observed in the photoluminescence owing to the Förster resonance energy transfer effect induced by dipole–dipole coupling. Following that, a BP‐based MIR photodetector is fabricated on the plasmonic metasurface. Here, the demonstrated BP/plasmonic metasurface‐based photodetector achieves the peak responsivity of 495.85 mAW−1 and ultrahigh operation speed (>10 MHz) at the power of 8.55 µW under 3.7 µm incident wavelength. This demonstrated photodetector opens a new opportunity for optoelectronic applications in the MIR region.

TDDFT versus GW/BSE Methods for Prediction of Light Absorption and Emission in a TADF Emitter.

Design concepts for organic light emitting diode (OLED) emitters, which exhibit thermally activated delayed fluorescence (TADF) and thereby achieve quantum yields exceeding 25%, depend on singlet-triplet splitting energies of order kT to allow reverse intersystem crossing at ambient temperatures. Simulation methods for these systems must be able to treat relatively large organic molecules, as well as predict their excited state energies, transition energies, singlet-triplet splittings, and absorption and emission cross sections with reasonable accuracy, in order to prove useful in the design process. Here we compare predictions of TDDFT with M06-2X and ωB97X-D exchange-correlation functionals and a GoWo@HF/BSE method for these quantities in the well-studied DPTZ-DBTO2 TADF emitter molecule. Geometry optimization is performed for ground state (GS) and lowest donor-acceptor charge transfer (CT) state for each functional. Optical absorption and emission cross sections and energies are calculated at these geometries. Relaxation energies are on the order of 0.5 eV, and the importance of obtaining excited state equilibrium geometries in predicting delayed fluorescence is demonstrated. There are clear trends in predictions of GoWo@HF/BSE, and TDDFT/ωB97X-D and M06-2X methods in which the former method favors local exciton (LE) states while the latter favors DA CT states and ωB97X-D makes intermediate predictions. GoWo@HF/BSE suffers from triplet instability for LE states but not CT states relevant for TADF. Shifts in HOMO and LUMO levels on adding a conductor-like polarizable continuum model dielectric background are used to estimate changes in excitation energies on going from the gas phase to a solvated molecule.

Study of optical absorption cross-section spectra and high-order harmonic generation of thymine, thymine glycol, and thymine dimer molecules.

The last two decades have witnessed advances in femtosecond and sub-femtosecond physics which have made accessible the study of phenomena with atomic and subatomic resolution. In particular, developments in laser physics and high-order harmonic generation have enabled tracking and investigating molecular reaction dynamics and the actual dynamics of electrons in chemical reactions, as well as investigating the structure of molecules. High-order harmonic generation has proved to be an ideal means for generating autosecond pulses. Time-dependent density functional theory provides an invaluable tool for the theoretical study of high-order harmonic generation. In this article, an exact study of the optical absorption cross-section spectra and high-order harmonic generation of thymine and its damage forms (thymine glycol and thymine dimer) has been performed using the time dependent density functional theory via the octopus code. The spectra have been characterized with the aim of distinguishing thymine from its damaged forms. The discrepancies in the optical absorption spectra of thymine and its damaged forms have been elucidated. The effects of laser pulse intensity and pulse profile on the high-order harmonic spectra of these molecules has been investigated, showing the possibility to distinguish between thymine and its damaged forms using high-order harmonic generation. In particular, it is demonstrated that high-order harmonic generation corresponding to a laser pulse with a cosinusoidal envelope and intensity [Formula: see text] clearly differentiates thymine from its damaged forms. This laser could potentially be applied to discern damage in more complex DNA material.

Study and analysis of the optical absorption cross section and energy states broadenings in quantum dot lasers

In this report, we measured experimentally the modal absorption spectra of the InP and InAsP quantum dot (QD) lasers using multi-section device technique. The optical absorption cross section (σ0) and inhomogeneous broadening for the ground state (GS) and excited state (ES) were analyzed and calculated theoretically from the absorption spectra. The results showed that the InP QD laser exhibited σ0 to be 1.347×10−14 ​cm2.eV and 3.016×10−14 ​cm2eV for GS and ES respectively, whereas for the InAsP QD material it was found as 0.511×10−14cm2eV and 3.099×10−14cm2.eV for GS and ES respectively. Moreover, the inhomogeneous broadening in the GS increases from 35.6 eV to 63.6 eV when As was added to InP QD, similarly, the inhomogeneous broadening of ES increases from 46.9 eV to 103.8 eV. The alloying InP QDs with arsenic decreases the σ0 of the ground state (lasing state) and increases both inhomogeneous and linewidth broadenings. This finding may help the grower to control the growth conditions and the molecule fractions of the crystal to improve the spectral properties of the optoelectronics devices.

Observation on the absorption of linearly polarized light by 133Cs atoms with optically resolved hyperfine structure

The absorption of linearly polarized light by 133Cs atoms is desired to be well known in many applications of atomic physics. For implementing this requirement, a simplified analytical form for the absorption of linearly polarized light by 133Cs atoms with optically resolved hyperfine structure is derived. Based on the obtained analytical form, optical absorption cross-sections of 133Cs atoms are simulated. An experiment is designed to further study the absorption of linearly polarized light by 133Cs atoms. Theory and experiment show that the influence of linearly polarized light on the distribution of 133Cs atoms will in turn affect the optical absorption. In order to demonstrate this kind of influence, an example for the measurement of the 133Cs vapor density is shown theoretically and experimentally, and a method of polynomial fit is proposed to reduce this influence.

Sources of Variability in the Response of Labeled Microspheres and B Cells during the Analysis by a Flow Cytometer.

A stochastic model of the flow cytometer measurement process was developed to assess the nature of the observed coefficient of variation (CV%) of the mean fluorescence intensity (MFI) from a population of labeled microspheres (beads). Several sources of variability were considered: the total number of labels on a bead, the path through the laser beam, the optical absorption cross-section, the quantum yield, the numerical aperture of the collection optics, and the photoelectron conversion efficiency of the photomultiplier (PMT) cathode. The variation in the number of labels on a bead had the largest effect on the CV% of the MFI of the bead population. The variation in the path of the bead through the laser beam was minimized using flat-top lasers. The variability in the average optical properties of the labels was of minor importance for beads with sufficiently large number of labels. The application of the bead results to the measured CV% of labeled B cells indicated that the measured CV% was a reliable measure of the variability of antibodies bound per cell. With some modifications, the model can be extended to multicolor flow cytometers and to the study of CV% from cells with low fluorescence signal.

Versatile Graphene‐Isolated AuAg‐Nanocrystal for Multiphase Analysis and Multimodal Cellular Raman Imaging†

Main observation and conclusionSurface‐enhanced Raman spectroscopy (SERS)‐based bioanalytical technique involves the interaction of SERS‐active substrate with complex environment, which has aroused intensive research interests. Compared to the commonly used Au SERS substrates, Ag nanocrystals have larger optical absorption cross section and acceptable price, but they possess poor oxidation resistance and potential biotoxicity, and the occurrence of unnecessary chemical reactions is inevitable due to the direct contact with probe molecules. Herein, we report a graphene‐isolated AuAg nanocrystal (GIAAN) with the SERS‐active AuAg core confined in a nanospace of few‐layer graphene shell, which possesses unique Raman peaks, high SERS activity, excellent stability, superior fluorescence quenching performance and good biocompatibility. Based on the limited solubility of GIAAN in water and organic solvents, it is able to spontaneously generate interfacial self‐assembled GIAAN (ISA‐GIAAN) film at immiscible two‐phase interfaces without any inducer, and multiphase Raman analysis of both water‐ and lipid‐soluble drug model molecules is further achieved. Moreover, the GIAAN is further non‐covalently functionalized with polyoxyethylenestearyl ether (C18‐PEG) to acquire GIAAN@PEG with good water‐solubility for SERS quantitative analysis in homogeneous system and multimodal Raman imaging of MCF‐7 cells. We expect the versatile GIAAN holds great potential to monitor drug metabolism and guide intended drug delivery in clinic trials.

Numerical assessment of transition metal nitrides nanofluids for improved performance of direct absorption solar collectors

Abstract Metallic nanostructures have attracted the attention for solar applications due to their high optical absorption cross-section, which is associated to the localized surface plasmon resonances phenomenon. In this work, the thermo-optical performance of a direct absorption solar collector using working-fluids composed of TiN, ZrN, and HfN nanospheres in an aqueous medium was evaluated using Mie theory and numerical solutions of a two-dimensional heat transfer model. The influence of nanoparticle size, volume fraction, collector height, and working-fluid temperature for the collector thermal and exergy efficiencies was investigated. Results revealed the transition metal nitrides nanoparticles have small scattering cross-sections and broad absorption peaks, matching well the solar radiation spectrum. The thermal efficiency is higher than 80% for collectors with all three nitrides nanoparticles. The collectors using TiN, ZrN, and HfN nanofluids exhibit thermal performance 50.4%, 36.5%, and 41.8%, respectively, higher than with Au nanofluid, which is a reference plasmonic material for solar applications. Moreover, collectors using transition metal nitrides reach higher outlet temperatures than with gold, leading to higher exergy efficiencies with low volume fraction. The maximum exergy efficiencies of DASC using TiN, ZrN, and HfN NF are 6.3%, 5.2%, and 5.6%, respectively, while using Au NF is 2.9%. The superior performance in a low-concentration regime allows the application of nanoparticles in the collector without increasing the required pumping work. The findings related to the photo-thermal efficiencies of transition metal nitride nanofluids, in addition to their low cost and high chemical stability, make them suitable to use for solar-related heating technologies.

Theoretical insight into the effect of Br, Na co-doping on electronic structure, photocatalytic and optical characteristics of g-C3N4 using first-principles and optical simulations

Solar photocatalysis splitting water for hydrogen production is a very hot issue nowadays, and great expectations are laid on g-C3N4 due to its low-cost, thermal stability, low toxicity. However, pure g-C3N4 is not perfect and it still has some shortcomings. By using first-principles calculations, Br and Na are considered being co-doped into the g-C3N4 to get whose photocatalytic performance further enhanced. By studying the electronic structure, optical properties, we find that doping Br Na atom can obviously narrow the band gap, and promote the optical absorption characteristics of g-C3N4, which means a better performance in photocatalysis. Furthermore, the optical absorption cross section and optical scattering cross section of two typical morphologies nano particle and nano rode are calculated by optical simulations to analyze the effects of doping on the optical absorption and scattering characteristics, which could provide another view in analyzing photocatalysis mechanisms and can make great difference on future photocatalyst designing.

Absorption cross section of gold nanoparticles based on NIR laser heating and thermodynamic calculations

We present a method for measuring the optical absorption cross section (sigma_{abs}) of gold nanoparticles (GNPs) based on optically heating the solution of GNPs with an 808 nm near-infrared (NIR) laser and measuring the temperature increase of the solution. We rely on the theoretical calculations based on the heat diffusion equations and experimental measurements based on the energy balance equations to measure the sigma_{abs} and the temperature distribution of single GNPs. Several morphologies, including gold nanospheres (GNSs), spherical gold nanoparticle conjugate (AuNPC), which are 20 nm GNSs surface-functionalized with an IR 808 dye, gold nanorods (GNRs), and gold nanourchins (GNUs), were studied. The study found that a single 20 nm GNS has the lowest sigma_{abs} and temperature distribution as compared to 100 nm GNUs. By increasing the size of GNSs from 20 to 30 nm, the magnitude of sigma_{abs} as well as temperature distribution increases by a factor of 5. The sigma_{abs} values of 20 and 30 nm GNSs calculated by Mie theory and the experimentally measured are in a good agreement. GNRs with equivalent radius (R_{eq}) 9.16 nm show the second lowest sigma_{abs}. By increasing the R_{eq} by a factor of 2 to 19.2 nm, the measured sigma_{abs} and temperature distribution also increased by a factor of 2. We also estimated sigma_{abs} for GNUs with diameters at 80 and 100 nm, which also have higher sigma_{abs} values. This work confirms that we can use temperature to accurately measure the sigma_{abs} of a variety of GNPs in solution.

Gold Nanorods as Contrast Agent for Photoacoustic Imaging (PAI) of Breast Cancer

Gold Nanorods (AuNRs) can play a fundamental role in the enhancement of photoacoustic imaging (PAI). This is due to their high and tunable optical absorption cross-section. Normally, the most convenient and highly stable AuNRs are synthesized using Cetyltrimethylammonium bromide (CTAB) as a surfactant. In this work, surface modification with poly ethylene glycol (PEGylation) and Silica coating (SiO2-coating) were applied to help in reducing toxicity of CTAB capped AuNRs. Breast cancer cell line (MDA-MB-231) was used to assess the modified AuNRs as contrast agents. A much brighter PAI of cells loaded with SiO2-coated AuNRs was obtained. Such an enhanced PA image is due to the larger cellular uptake, as observed from bright field microscope images taken for the cells. On the other hand, the corresponding PA image for cells with PEGylated AuNRs is diminished. The increase in cell uptake, in case of SiO2-coated AuNRs, could result due to the adsorption of protein, contained in the cell culture media, on the negatively charged silica surface, which increases intercellular uptake by receptor-mediated endocytosis. Moreover, the increase of the brightness of PAI of SiO2-coated AuNRs may be due to the lowering of the interfacial thermal resistance between gold and the surrounding solvent. This allows the particle to release more heat to its environment which increases the generated acoustic signals from these particles.