Gold Nanoparticle Films Research Articles

Nanoparticle-Modified Apoferritin Nanotransfer for Targeted Cytostatic Transport.

Ferritin is a globular intracellular protein that acts as the main reservoir for iron. Malignancies are associated with increased plasma ferritin concentrations. A number of studies show that tumor cells express high levels of transferrin receptors (TfR). Increased TfR expression was observed in prostate carcinoma. Apoferritin (APO) can be used as a protein nanotransporter into which a suitable medicinal substance can be encapsulated. Nanoparticles increase the permeability of tumor cells to nanotransporters and have a photothermal effect. The aim of this study was to encapsulate doxorubicin (DOX) into APO and to modify the resulting APO/DOX with gold (AuNPs) and silver nanoparticles prepared by green synthesis (AgNPsGS). APO was characterized using 10% sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) - 120 V, 60 min, 24 mM Tris, 0.2 M glycine, 3 mM SDS. DOX fluorescence (Ex 480 nm; Em 650 nm) was observed, with a typical absorption maximum at 560 nm. Electrochemical measurement was performed in Brdicka solution (three-electrode setup). AgNPsGS were prepared by green synthesis using clover (Trifolium pratense L.). An electrophoretic study of APO and APO/DOX (5-100 μg/mL) was performed and the behavior of APO and APO/DOX (10 μM) as a function of pH was monitored. In an acidic environment, APO forms subunits of about 20 kDa; in an alkaline medium, it forms a globular protein of about 450 kDa. A change in APO/DOX mobility (about by 10%) was observed. A film of gold nanoparticles was applied to the APO/DOX surface. APO/DOX-AuNPs were washed with ultra-pure water. pH-dependent release of DOX a was monitored. The amount of DOX analyzed was increased by up to 50%. Furthermore, an AgNPsGS-DOX complex (1 mg AgNPsGS/100 μM DOX) was generated and prepared. Subsequently, the AgNPsGS-DOX complex was encapsulated into APO. To further improve therapeutic efficacy, the APO/AgNPsGS-DOX complex was coated with an Au layer. APO/AgNPsGS-DOX/AuNPs were stable and DOX was released from the complex after physical parameters had changed. APO nanocomplexes were prepared and modified to increase therapeutic efficacy against tumors. Tumor cell targeting was achieved by binding to TfR and via increased tumor cell permeability and retention. Release of the drug was made possible due to a pH change and photothermal activation that will now be tested. This work was supported by COST European Cholangiocarcinoma Network CA18122 and International Collaboration Project of The European Technology Platform for Nanomedicine. The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study. The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers. Submitted: 21. 3. 2019 Accepted: 14. 5. 2019.

Improving direct immunoassay response by layer-by-layer films of gold nanoparticles – Antibody conjugate towards label-free detection

The conjugation of nanoparticles with antibodies has been successfully applied in sandwich immunoassays for detecting cancer biomarkers. However, two antibodies are necessary to perform such experiment, being one of them functionalized with a signal label for optical or electrochemical assay. This approach is time and cost consuming compared to direct label-free immunoassays. In this study, we propose the synthesis of gold nanoparticles conjugated to anti-PSA antibody to produce a label-free impedimetric immunosensors based on nanostructured Layer-by-Layer (LbL) films. Prostate-specific antigen (PSA) detection was performed by electrochemical impedance spectroscopy demonstrating a detection mechanism governed by Langmuir-Freundlich adsorption model. This strategy provided a significant sensitivity using 10-fold less antibody than conventional immunosensors, i.e. decreasing costs using a simple approach, with a limit of detection of 0.17 ng mL−1 and an analytical range of 0.1–20 ng mL−1 indicating that our sensor is potentially useful for clinical applications.

Plasma jet printing for preparation of N-doped graphene electrode

Graphene, the 2-D allotrope of carbon, is well known for its remarkable electronic and optical properties. Nitrogen doping can manipulate local electronic structure and thus improve the performance of a graphene powered device. Therefore, developing a method of graphene production that is low in cost, simple in operation and high in controllability is important. It carries a vital value for the industrial applications of graphene materials in the future. For this study, graphene oxide (GO) and GO/AuNPs (gold nanoparticles) films were prepared using plasma jet printing technology. Then, a dielectric barrier discharge at a sub-atmospheric pressure in nitrogen gas was used to obtain N-doped and reduced graphene oxide (N-rGO). The microstructure of the sample was observed using a scanning electron microscope. Then, the materials were analyzed using X-ray diffraction, Raman spectra, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Finally, the cyclic voltammetry profiles and galvanostatic charge/discharge curves of N-rGO and N-rGO/AuNPs capacitors were tested. Results have indicated that the ultimate materials with a porous structure after the nitrogen plasma treatment presented a pretty good capacitor performance.

Nonlinear Optical Studies of Gold Nanoparticle Films.

Gold films are widely used for different applications. We present the results of third- and high-order nonlinear optical studies of the thin films fabricated from Au nanoparticle solutions by spin-coating methods. These nanoparticles were synthesized by laser ablation of bulk gold in pure water using 200 ps, 800 nm pulses. The highest values of the nonlinear absorption coefficient (9 × 10−6 cm W−1), nonlinear refractive index (3 × 10−11 cm2 W−1), and saturation intensity (1.3 × 1010 W cm−2) were achieved using 35 fs, 400 nm pulses. We also determined the relaxation time constants for transient absorption (220 fs and 1.6 ps) at 400 nm. The high-order harmonic generation was studied during propagation of 35 fs, 800 nm pulses through the plasma during the ablation of gold nanoparticle film and bulk gold. The highest harmonic cutoff (29th order) was observed in the plasma containing gold nanoparticles.

Plasma-Ice Interface as Thermodynamically Size-Tunable Reaction Field: Development of Plasma-Assisted Freeze Templating.

Interfaces or interfacial layers, such as gas-liquid interfaces, are critical for many physical and chemical reactions and are utilized for designing a wide range of materials. In this study, we propose a plasma-assisted freeze templating (PFT) method for materials processing. It uses a new type of interfacial reaction field, i.e., plasma-ice interface. In PFT, a micro- or nanoscale liquid layer formed on the ice body of a frozen aqueous solution is used as a reaction field in which the solutes are highly enriched and the chemical reactions are initiated by reactive species from the plasma. We demonstrated the synthesis of a self-standing gold nanoparticle (AuNP) film of porous structure by PFT in which a helium cryoplasma jet was irradiated onto a frozen solution of auric ions. This PFT method accomplished a surfactant-free and area-selective synthesis of a AuNP film and was unique in comparison with the conventional chemical synthesis of nanostructured gold materials. Furthermore, simple control of the AuNP film was demonstrated by tuning the thickness of the thin liquid layer. This was done by changing the temperature or concentration of the aqueous solution. PFT was demonstrated as a thermodynamically size-tunable scheme for material design; it exploits the plasma-ice interface and is expected to become a novel technique for a wide range of micro- and nanoengineering applications.

Seed-Mediated Electroless Deposition of Gold Nanoparticles for Highly Uniform and Efficient SERS Enhancement.

A seed-mediated electroless deposition (SMED) approach for fabrication of large-area and uniform gold nanoparticle films as efficient and reproducible as surface-enhanced Raman scattering (SERS) substrates was presented. This approach involved a seeding pretreatment procedure and a subsequent growth step. The former referred to activation of polylysine-coated glass slides in gold seed solution, and the latter required a careful control of the reactant concentration and reaction time. With the aid of gold seeds and appropriate reaction conditions, a large-area and uniform nanofilm with evenly distributed gold nanoparticles (Au NPs) was formed on the surface of the substrates after adding a mixed solution containing ascorbic acid and trisodium citrate. The morphology of the Au nanofilm was examined by scanning electron microscopy. The size evolution of Au NPs on the surface of the substrates was analyzed in detail. The nanofilm substrate was prepared by reaction conditions of the seeded activation process: 10 mL ascorbic acid and trisodium citrate mixture and 30 min of soaking time, which exhibited an excellent uniformity and reproducibility of SERS enhancement with relative standard deviation (RSD) values of less than 8% (particularly, a RSD value of 3% can be reached for the optimized measurement). Compared to the common electroless deposition, the seed-mediated electroless deposition possessed inherent advantages in controllability, reproducibility, and economic benefit.

Layer-by-Layer Films of Graphene Nanoplatelets and Gold Nanoparticles for Methyl Parathion Sensing

The wide use of pesticides in agriculture makes necessary the development of cost-effective, easy, and rapid methods aiming for environmental and health protection. We propose the fabrication and investigation of layer-by-layer (LbL) films composed of reduced graphene oxide stabilized in polyelectrolytes in the presence and absence of gold nanoparticles (AuNPs) toward methyl parathion (MP) detection. In addition to the film assembly and sensing application, we performed a systematic investigation of the influence of MP distribution on the limit of detection and linear range of the sensor. Indium tin oxide (ITO) electrodes modified with the LbL films were applied by differential pulse voltammetry achieving an LOD and a linear range of 0.226 ppm and 0.25–40 ppm in the absence of AuNPs, while in the their presence, a wider linear range was achieved (0.5–60 ppm, with LOD of 0.770 ppm). Real sample analysis indicated potential application in water, ground, and vegetables, with recovery varying from 92% to 113%.

Influence of antibody immobilization strategy on carbon electrode immunoarrays.

We report here the influence of antibody immobilization strategy for protein immunosensors on screen printed carbon electrode arrays in terms of antibody binding activity, analytical sensitivity, limit of detection, and stability. Horseradish peroxidase (HRP) was the model analyte with anti-HRP immobilized on the sensors, and HRP activity was used for detection. Covalently immobilized anti-HRP antibodies on electrodes coated with chitosan, electrochemically reduced graphene oxide (rGO), and dense gold nanoparticle (AuNP) films had only 20-30% of the total immobilized antibodies active for binding. Active antibodies increased to 60% with passively adsorbed antibodies on bare electrodes, to 85% with oriented antibodies using protein A covalently immobilized on AuNP-coated carbon electrode, and to 98% when attached to protein A passively adsorbed onto bare electrodes. Passively adsorbed antibodies on bare electrodes lost activity in 1-2 days, but antibodies immobilized using other strategies remained relatively stable after 5 days. Covalent immobilization gave limits of detection (LOD) of 40 fg mL-1, while passively adsorbed antibodies or protein A on carbon electrodes had LODs 4-8 fg mL-1, but were unstable. Sensitivity was highest for antibodies covalently attached to AuNP electrodes (2.40 nA per log pg per mL) that also had highest antibody coverage, and decreased slightly when protein A on AuNP was used to orient antibodies. Passively adsorbed antibodies and oriented antibodies on protein A gave slightly lower sensitivities. Immobilization strategy or antibody orientation did not have a significant effect on LOD, but dynamic range increased as the number of active antibodies on sensor surfaces increased.

Surface plasmon resonance “hot spots” and near-field enhanced spectroscopy at interfaces

Optical excitations and mutual couplings of surface plasmons with specific modes in metal nanostructures are the physical basis for developing the high spatial resolution, high sensitivity, and high precision spectroscopy. Here, we systematically review latest advances in optical excitations, classifications and identifications of surface plasmon resonance modes and their typical applications in several typical interfaces. We discuss several aspects below. First, the intrinsic mechanism of creating " hot spots” in metal particle-film systems is elucidated by the finite-difference time-domain numerical method. Spatial transfers and influence factors of the " hot spots” under plasmon-induced electric- resonance and plasmon-induced magnetic-resonance conditions are discussed. Second, the plasmon-induced magnetic-resonance in the visible-light region is successfully realized in a gold nanoparticle-film system. Meanwhile, experimental results of surface-enhanced Raman spectroscopy show that the " hot spots” in the magnetic-resonance mode can output Raman scattering with a much higher enhancement factor than that in the conventional electric-resonance mode. Third, we design nonlinear nanorulers that can reach approximately 1-nm resolution by utilizing the mechanism of plasmon-enhanced second-harmonic generation (PESHG). Through introducing Au@SiO<sub>2</sub> (core@shell) shell isolated nanoparticles, we strive to maneuver electric-field-related gap modes such that a reliable relationship between PESHG responses and gap sizes, represented by " PESHG nanoruler equation”, can be obtained. Fourth, a critical and general solution is proposed to quantitatively describe the spatial resolution and directional emission in tip-enhanced Raman spectroscopy and tip-enhanced fluorescence. These results may help enhance our understanding of the intrinsic physical mechanism of the surface plasmon resonance, and offer opportunities for potential applications in surface-enhanced Raman spectroscopy, tip-enhanced Raman spectroscopy, second harmonic generation, and other plasmon-enhanced spectroscopy.

Solvent-induced modulation of the chemical sensing performance of gold nanoparticle film chemiresistors

The analyte sensitivity of a chemical sensor is commonly tuned by the type of chemical recognition element. Here, it is shown that the solvent also plays a critical role in the sensing performance of thiol-capped gold nanoparticle film chemiresistors. Both the resistance change and speed of response of the chemiresistor can be optimised by adjusting the composition of methanol and water used for detection of the pesticide permethrin. It is shown that the resistance change is governed by the partitioning of the analyte between the thiol coating of the gold nanoparticles and the solvent. The partition coefficient, log P, has a strong influence on the response magnitude and is directly correlated to the response time. By varying the solvent, the log P of extremely high partitioning analytes such as permethrin in water can be lowered, and a much larger resistance change can be obtained in a shorter time period (5 min). A theoretical model is developed that correlates the experimental findings to changes in analyte partition coefficient with solvent type and composition. The results have significant implications for the design of chemical sensors and solvent conditions that can offer an enhanced signal within a practical timeframe.

Electrodeposition⁻Assisted Assembled Multilayer Films of Gold Nanoparticles and Glucose Oxidase onto Polypyrrole-Reduced Graphene Oxide Matrix and Their Electrocatalytic Activity toward Glucose.

The study reports a facile and eco-friendly approach for nanomaterial synthesis and enzyme immobilization. A corresponding glucose biosensor was fabricated by immobilizing the gold nanoparticles (AuNPs) and glucose oxidase (GOD) multilayer films onto the polypyrrole (PPy)/reduced graphene oxide (RGO) modified glassy carbon electrode (GCE) via the electrodeposition and self-assembly. PPy and graphene oxide were first coated on the surface of a bare GCE by the electrodeposition. Then, AuNPs and GOD were alternately immobilized onto PPy-RGO/GCE electrode using the electrodeposition of AuNPs and self-assembly of GOD to obtain AuNPs-GOD multilayer films. The resulting PPy-RGO-(AuNPs-GOD)n/GCE biosensors were used to characterize and assess their electrocatalytic activity toward glucose using cyclic voltammetry and amperometry. The response current increased with the increased number of AuNPs-GOD layers, and the biosensor based on four layers of AuNPs-GOD showed the best performance. The PPy-RGO-(AuNPs-GOD)4/GCE electrode can detect glucose in a linear range from 0.2 mM to 8 mM with a good sensitivity of 0.89 μA/mM, and a detection limit of 5.6 μM (S/N = 3). This study presents a promising eco-friendly biosensor platform with advantages of electrodeposition and self-assembly, and would be helpful for the future design of more complex electrochemical detection systems.

Tuning the interparticle distances in self-assembled gold nanoparticle films with their plasmonic responses

This study presents the fabrication and characterization of imine-functionalized Au nanoparticle (Au NP) film using Poly (ethyleneimine) (PEI) as a reducing and surface stabilizing reagent. The PEI-capped Au NPs can be self-assembled at the water-hexane interface by adding alcohol containing alkanethiol. Furthermore, large-area film made from PEI and alkanethiol-capped Au NPs is produced on a glass substrate with varying interparticle distances that can be controlled by the alkyl chain lengths, ranging from C2 (ethanethiol), C4 (1-butnaethiol), C6 (1-hexanethiol), C8 (1-octanethiol), to C10 (1-decanethiol) at the water-hexane interface. These 2D films show strong collective localized surface plasmon resonance (LSPR) extinction bands that are tunable via near-field coupling of adjacent Au NPs. Pronounced chain-length-dependent red shifts of the SPR bands have been observed for the PEI-capped Au films. The obtained PEI-capped Au films exhibit distinct interparticle-distance-dependent SERS characteristics.

Spontaneous self-assembly of conductive molecular linkages between gold nanoelectrodes from aryl diisocyanides

The adsorption of 4,4′-biphenyldiisocyanide (BPDI) and 4,4″-terphenyldiisocyanide (TPDI) is studied on gold films from solution as a function of coverage using attenuated total internal reflection infrared (ATR-IR) spectroscopy to identify the nature and determine the orientation of the adsorbed species. At low coverages, both BPDI and TPDI adsorb with their molecular planes parallel to the surface and exhibit a single isocyanide stretching frequency consistent with the formation of gold-linked BPDI and TPDI oligomer chains, analogous to similar oligomers found when using 1,4-phenylene diisocyanobenzene (1,4-PDI) on gold. A vibrational mode due to a free isocyanide appears at higher BPDI and TPDI exposures, indicating that adsorbed BPDI and TPDI have reoriented to form η1-adsorbed species, in a similar behavior to that found previously for 1,4-PDI on gold. It has been demonstrated that gold-PDI oligomer chains provide conductive bridges between gold nanoparticles on mica, in which the conductivity initially increases with 1,4-PDI dose as the number of oligomer linkages increases, but then decreases once again as the 1,4-PDI adopts a perpendicular orientation to disrupt the conductive chains. Similar behavior is found for BPDI and TPDI, where the dose at which a conductivity decrease is noted corresponds to the appearance of the free isocyanide stretching mode in the ATR-IR spectra. The sheet resistance R of the conductive films was measured as a function of sample temperature where it was found to vary linearly as $${\text{ln}}(R)$$ versus $$1/\sqrt T$$. The slopes of such plots for gold nanoparticle films with different initial conductivities are used to estimate the tunneling barrier for gold nanoparticles linked by BPDI- and TPDI-derived oligomers. The results suggest that the barrier increases in the order TPDI > BPDI > 1,4-PDI. These results indicate that the ability to link between gold nanoparticles on surfaces by forming gold-containing oligomer chains with phenyl-linked diisocyanide is a quite general phenomenon.

Sensitivity-Enhanced Optical Fiber Biosensor Based on Coupling Effect Between SPR and LSPR

An optical fiber biosensor based on gold nanoparticles and protein A co-modified Au film coated photonic crystal fiber (Au-PCF) is proposed and demonstrated for human IgG detection. For the electric field coupling effect between surface plasmon resonance (SPR) of Au film and localized surface plasmon resonance (LSPR) of gold nanoparticles, the refractive index sensitivity can be enhanced significantly. Besides, goat anti-human IgG is immobilized by protein A modified on the gold nanoparticles surface to interact with human IgG. Protein A can specifically bind the Fc region of the antibodies and has a high degree of orientation for capturing antibodies. Experimental results indicate that the refractive index sensitivity of Au-PCF sensor modified gold nanoparticles reaches 3915 nm/RIU, which is about 1.6 times higher than the Au-PCF sensor without gold nanoparticles modified. The lowest detection limit of human IgG based on gold nanoparticles and protein A co-modified Au-PCF sensor is 37 ng/mL, which is about 2.4 times and 6.3 times lower than gold nanoparticles-Au-PCF sensor and Au-PCF sensor, respectively. Such a sensitivity enhanced optical fiber biosensor based on SPR-LSPR coupling effect has potential application in clinical disease diagnosis and immunoassays.

Fabrication of Strain Gauges via Contact Printing: A Simple Route to Healthcare Sensors Based on Cross-Linked Gold Nanoparticles

In this study, we developed a novel and efficient process for the fabrication of resistive strain gauges for healthcare-related applications. First, 1,9-nonanedithiol cross-linked gold nanoparticle (GNP) films were prepared via layer-by-layer (LbL) spin-coating and subsequently transferred onto flexible polyimide foil by contact printing. Four-point bending tests revealed linear response characteristics with gauge factors of ∼14 for 4 nm GNPs and ∼26 for 7 nm GNPs. This dependency of strain sensitivity is attributed to the perturbation of charge carrier tunneling between neighboring GNPs, which becomes more efficient with increasing particle size. Fatigue tests revealed that the strain-resistance performance remained nearly the same after 10.000 strain/relaxation cycles. We demonstrate that these sensors are well suited to monitor muscle movements. Furthermore, we fabricated all-printed strain sensors by directly transferring cross-linked GNP films onto soft PDMS sheets equipped with interdigitated electrodes. Due to the low elastic modulus of poly(dimethylsiloxane) (PDMS), these sensors are easily deformed and, therefore, they respond sensitively to faint forces. When taped onto the skin above the radial artery, they enable the well-resolved and robust recording of pulse waves with diagnostically relevant details.

Highly Sensitive Colorimetric Detection of Early Stage Aluminum Corrosion in Water Using Plasmonic Gold Nanoparticle Films

AbstractCorrosion remains the major reason for the failure of metallic parts. Therefore, spotting and evaluating the severity of corrosion at its early stages is of great importance. Herein, a novel optical corrosion probe based on plasmonic gold nanoparticles immobilized on aluminum surface is demonstrated. The measurements confirm that the corrosion rate and mechanism of bare aluminum is identical to that of the developed probe. During Al corrosion, the gold nanoparticles are located entirely at the interface between the inner dense and outer porous layers of the formed aluminum oxide. The growth of the inner oxide layer during corrosion prompts a change in the reflection spectra and the appearance of strong coloration that is easily observed by the naked eye. Numerical modeling reveals that these optical changes occur mainly due to light interference within the inner oxide layer, which is located between the plasmonic gold nanoparticles and the underlying reflective aluminum. The ability of the plasmonic probe is further demonstrated to monitor the aqueous corrosion of thin films, bulk and alloyed Al, as well as Al protected by a self‐assembled monolayer of hydrophobic surfactant, and, hence, the wide applicability of the method.

Microporous Plasmonic Capsules as Stable Molecular Sieves for Direct SERS Quantification of Small Pollutants in Natural Waters

AbstractFast and versatile optical SERS methods represent a major advance in chemical analysis of environmental samples such as water. To date, however, these ultrasensitive methods are hindered by two key drawbacks: (i) colloidal stability and (ii) chemical diversity, both arising from the compositional complexity of natural samples. Here, we present an engineered material that, due to its unique microporous structure, imparts colloidal stability and provides selectivity while confining a densely populated film of gold nanoparticles optimized for the generation of large electromagnetic fields. The material is tested against natural water for the ultraquantification of dichlorodiphenyl‐trichloroethane (DDT), a ubiquitous environmental pollutant.

Highly sensitive gas sensor using hierarchically self-assembled thin films of graphene oxide and gold nanoparticles

In this study, we fabricated hierarchically self-assembled thin films composed of graphene oxide (GO) sheets and gold nanoparticles (Au NPs) using the Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) techniques and investigated their gas-sensing performance. First, a thermally oxidized silicon wafer (Si/SiO2) was hydrophobized by depositing the LB films of cadmium arachidate. Thin films of ligand-capped Au NPs and GO sheets of the appropriate size were then sequentially transferred onto the hydrophobic silicon wafer using the LB and the LS techniques, respectively. Several different films were prepared by varying the ligand type, film composition, and surface pressure of the spread monolayer at the air/water interface. Their structures were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and their gas-sensing performance for NH3 and CO2 was assessed. The thin films of dodecanethiol-capped Au NPs and medium-sized GO sheets had a better hierarchical structure with higher uniformity and exhibited better gas-sensing performance.

Tuning Metamaterials Nanostructure of Janus Gold Nanoparticle Film for Surface-Enhanced Raman Scattering

Large-area metamaterial nanostructures of a Janus gold nanoparticle (AuNP) film decorated with thiol-terminated polymers have been fabricated and tuned at an oil/water interface by a facile and innovative process. The results show that AuNPs had been arrayed with ultrasmall gaps between neighboring particles laminated with polymer which is called a Janus film. This film exhibits a sensitive property for surface-enhanced Raman scattering (SERS) for determination of both hydrophilic methylene blue and hydrophobic thiram. The structure-dependent SERS of the Janus AuNP film has also been confirmed by a finite-difference time domain (FDTD) method.

Mechanism of large optical nonlinearity in gold nanoparticle films.

The Z-scan technique, using femtosecond (fs) laser pulses at 1480nm laser pulses, was used to measure the nonlinear optical properties of gold (Au) nanoparticle (NP) films made by both nanosecond (ns) and fs pulsed laser deposition (PLD) in vacuum. At irradiance levels of 1×1012 Wm-2, the ns-PLD films displayed induced absorption with β=4×10-5 mW-1, and a negative lensing effect with n2=-4.7×10-11 m2 W-1 with somewhat smaller values for the fs-PLD films. These values of n2 imply an unphysically large change in the real part of the refractive index, demonstrating the need to take account of nonlinear changes of the Fresnel coefficients and multiple beam interference in Z-scan measurements on nanoscale films. Following this approach, the Z-scan observations were analyzed to determine the effective complex refractive index of the NP film at high irradiance. It appears that at high irradiance the NP film behaves as a metal, while at low irradiance it behaves as a low-loss dielectric. Thus, it is conjectured that, for high irradiance near the waist of the Z-scan laser beam, laser driven electron tunneling between NPs gives rise to metal-like optical behavior.