Fundamental examination of nanoparticle heating kinetics upon near infrared (NIR) irradiation

Near infrared (NIR) light, which spans wavelengths from ~700-1100 nm holds particular promise in bionanotechnology-enabled applications because both NIR light and nanoparticles (NPs) have the potential for remote activation leading to exquisite localization and targeting scenarios. In this study, aqueous solutions of carbon and metal-based NPs (carbon black, single-walled carbon nanotubes, silver nanoparticles and copper nanoparticles) were exposed to continuous NIR laser (λ = 1064 nm) irradiation at powers of 2.2W and 4.5W. The differential heating of bulk aqueous suspension of NPs with varying physicochemical properties revealed maximum temperatures of 67 °C with visible evidence of condensation and bubble formation. The basis of the NP heating is due to the strong intrinsic optical absorbance in the NIR spectral window and the transduction of this NIR photon energy into thermal energy. In this regard, UV-vis measurements can accurately predict NP heating kinetics prior to NIR irradiation. Further, a uniform thermodynamic heating model demonstrates close agreement with the experimental data for the low NIR-absorbing NPs. However, the uniform thermodynamic heating model used in this study does not accurately portray the energy release upon localized NP heating because of bubble formation for the highly absorbing NPs. Therefore, this study reveals the differential heating kinetics of NPs excited with NIR with implications in the development of novel NIR-NP-based systems.     

Near Infrared Light‐Triggered Drug Generation and Release from Gold Nanoparticle Carriers for Photodynamic Therapy

A photoprecursor Pc 227 is covalently bound onto gold nanoparticles (Au NPs) to produce the known photodynamic therapy (PDT) drug Pc 4 upon 660 nm photoirradiation. The photochemical formation of the photoproduct Pc 4 is identified by spectroscopy, chromatography, and mass spectrometry and its PDT efficacy is equal to Pc 4 when administered non‐covalently by Au NPs, with the added benefit of improved covalent delivery and targeted NIR‐triggered release from the covalent Pc 227‐Au NP conjugate, while during transport the attached Pc 227 is quenched by the Au NP and PDT inactivated.     

Probing dynamically tunable localized surface plasmon resonances of film-coupled nanoparticles by evanescent wave excitation

The localized surface plasmon resonance (LSPR) spectrum associated with a gold nanoparticle (NP) coupled to a gold film exhibits extreme sensitivity to the nanogap region where the fields are tightly localized. The LSPR of an ensemble of film-coupled NPs can be observed using an illumination scheme similar to that used to excite the surface plasmon resonance (SPR) of a thin metallic film; however, in the present system, the light is used to probe the highly sensitive distance-dependent LSPR of the gaps between NPs and film rather than the delocalized SPR of the film. We show that the SPR and LSPR spectral contributions can be readily distinguished, and we compare the sensitivities of both modes to displacements in the average gap between a collection of NPs and the gold film. The distance by which the NPs are suspended in solution above the gold film is fixed via a thin molecular spacer layer and can be further modulated by subjecting the NPs to a quasistatic electric field. The observed LSPR spectral shifts triggered by the applied voltage can be correlated with angstrom scale displacements of the NPs, suggesting the potential for chip-scale or flow-cell plasmonic nanoruler devices with extreme sensitivity.     

Polymer-assisted fabrication of gold nanoring arrays

In this paper,we report a new strategy for the fabrication of gold nanoring arrays via colloidal lithography and polymer-assisted self-assembly of gold nanoparticles (Au NPs).First,multi-segmented polymer nanorod arrays were fabricated via colloidal lithography.They were then used as templates for Au NP adsorption,which resulted in nanoparticles on the poly(4-vinyl pyridine) (P4VP) segments.Continuous gold nanorings were formed after electroless deposition of gold.The diameter,quantity,and spacing of the gold nanorings could be tuned.Three dimensional coaxial gold nanorings with varying diameters could be fabricated on a polymer nanorod by modifying the etch parameters.The nanorings exhibited optical plasmonic resonances at theoretically predicted wavelengths.In addition,the polymer-assisted gold nanorings were released from the substrate to generate a high yield of free-standing nanorings.This simple,versatile method was also used to prepare nanorings from other metals such as palladium.     

Concentrated colloids of silica-encapsulated gold nanoparticles: colloidal stability, cytotoxicity, and X-ray absorption

As an effort to develop a new, effective, nontoxic X-ray contrast agent, the concentrated colloids of silica-encapsulated gold nanoparticles (Au@SiO2 NPs) were fabricated and their colloidal stability, cytotoxicity, and X-ray absorption were investigated. The concentrated colloidal NPs were manufactured by forming a 4 nm-thick silica shell on the surface of each Au NP with 15 nm diameter, followed by enrichment to [Au] = 100 mM. They were very stable in water: the NPs were well separated each other without forming agglomerates and their optical property was very similar to that before enrichment. The colloidal stability of the NPs in biological environment was strongly dependent on their previous morphology in water. The NPs with minor shell damage were stable in phosphate buffered saline (PBS) solution: both in water and in PBS solution, they showed very similar morphology and optical property. However, the NPs with profound shell damage formed big agglomerates in PBS solution, resulting in the red-shift and broadening of the Au surface plasmon resonance peak. Cell viability and proliferation assessments revealed the biocompatibility of the Au@SiO2 NPs: no apparent cytotoxicity was observed even at 100 ppm NPs. The concentrated colloidal NPs showed very strong X-ray absorption. Their relative X-ray transmittance to water was comparable to that of a commercial agent. Considering these, the concentrated colloids of the Au@SiO2 NPs are suitable for an X-ray contrast agent.     

Localized surface plasmon resonance of single silver nanoparticles studied by dark-field optical microscopy and spectroscopy

Localized surface plasmon resonance (LSPR) of Ag nanoparticles (NPs) with different shapes and disk-shaped Ag NP pairs with varying interparticle distance is studied using dark-field optical microscopy and spectroscopy (DFOMS). Disk-, square-, and triangular-shaped Ag NPs were fabricated on indium tin oxide-coated glass substrates by electron beam lithography. The LSPR spectra collected from single Ag NPs within 5×5 arrays using DFOMS exhibited pronounced redshifts as the NP shape changed from disk to square and to triangular. The shape-dependent experimental LSPR spectra are in good agreement with simulations using the discrete dipole approximation model, although there are small deviations in the peak wavelengths for square- and triangular-shaped NPs. The LSPR spectra of disk-shaped Ag NP pairs with varying interparticle distances were acquired from five different locations across the pair axis. It was clearly observed that the LSPR wavelength redshifts as the interparticle distance decreases, indicating a strong interaction when two Ag NPs are close to each other.     

Ethanol Oxidation via 12-Electron Pathway on Spiky Au@AuPd Nanoparticles Assisted by Near-Infrared Light

In this work, ethanol oxidation reaction (EOR) via 12-electron (C1-12e) pathway on spiky Au@AuPd nanoparticles (NPs) with ultrathin AuPd alloy shells is achieved in alkaline media with the assistance of the near-infrared (NIR) light. It is found that OH radicals can be produced from the OH_ads species adsorbed on the surfaces of Pd atoms led by surface plasmon resonance (SPR) effect of spiky Au@AuPd NPs under the irradiation of NIR light. Moreover, OH radicals play the key role for the achievement of EOR proceeded by the desirable C1-12e pathway because OH radicals can directly break the C–C bonds of ethanol. Accordingly, the electrocatalytic performance of spiky Au@AuPd NPs toward EOR under NIR light is greatly improved. For instance, their mass activity can be up to 33.2 A mg_pd⁻¹ in the 0.5 m KOH solution containing 0.5 m ethanol, which is about 158 times higher than that of commercial Pd/C catalysts (0.21 A mg_pd⁻¹) and is better than those of the state-of-the-art Pd-based catalysts reported in literature thus far, to the best of our knowledge. Moreover, their highest mass activity can be further improved to 118.3 A mg_pd⁻¹ in the 1.5 m KOH solution containing 1.25 m ethanol.     

Controlling the aggregation behavior of gold nanoparticles

The aggregation of Au nanoparticles (NPs) in solution is influenced by cationic and oligocationic species. The polarization of the conduction electron oscillations in adjacent gold nanoparticles causes a new red-shifted plasmon absorbance attributed to the coupling of the plasmon absorbance of the particles. This appearance of an additional plasmon band is of particular interest to the field of SERS and has led to research works directed at the stabilization of small colloid aggregates in solution. The surface plasmon coupling can be tuned by controlling the aggregation of gold nanoparticles by the addition of some “cross-linking” agent. Here we develop a simple method to fabricate linear-chainlike aggregates of gold nanoparticles (so-called nanochains), tuning the linear optical properties in a wide wavelength range from visible to the near-infrared. The aggregation behavior and linear self-assembly mechanism of citrate-stabilized gold colloids as provoked by the addition of cetyltrimethylammonium bromide (CTAB) and 11-mercaptoundecanoic acid (MUA) are also analyzed. The line-assembly mechanism of gold nanochain is attributed to the preferential binding of CTAB molecules on a certain facet of gold NPs and the Au NP electrostatic interactions. We also found that the 11-mercaptoundecanoic acid was effective to prevent the further aggregation of CTAB-modified gold colloids.     

Eco-friendly (green) synthesis of magnetically active gold nanoclusters

Abstract

Au–Fe_xO_y composite nanoparticles (NPs) are of great technological interest due to their combined optical and magnetic properties. However, typical syntheses are neither simple nor ecologically friendly, creating a challenging situation for process scale-up. Here we describe conditions for preparing Au–Fe_xO_y NPs in aqueous solutions and at ambient temperatures, without resorting to solvents or amphiphilic surfactants with poor sustainability profiles. These magnetic gold nanoclusters (MGNCs) are prepared in practical yields with average sizes slightly below 100 nm, and surface plasmon resonances that extend to near-infrared wavelengths, and sufficient magnetic moment (up to 6 emu g^–1) to permit collection within minutes by handheld magnets. The MGNCs also produce significant photoluminescence when excited at 488 nm. Energy dispersive X-ray (EDX) analysis indicates a relatively even distribution of Fe within the MGNCs, as opposed to a central magnetic core.     

Nanocomposite synthesis by absorption of nanoparticles into macroporous hydrogels. Building a chemomechanical actuator driven by electromagnetic radiation

Macroporous hydrogels irreversibly absorb solid nanoparticles from aqueous dispersions. A nanocomposite is made using a macroporous thermosensitive hydrogel (poly(N-isopropylacrylamide-co-(2-acrylamido-2-methyl propane sulfonic acid)) (poly(NIPAm-co-AMPS)) and conductive polymer (polyaniline, PANI) nanoparticles (PANI NPs). Macroporous gels of poly(NIPAm-co-AMPS) were made by a cryogelation technique. NPs of PANI were produced by precipitation polymerization. It is found that PANI NPs are easily absorbed into the macroporous hydrogels while conventional non-porous hydrogels do not incorporate NPs. It is shown that PANI NPs, dispersed in water, absorb NIR laser light or microwave radiation, increasing their temperature. Upon irradiation of the nanocomposite with microwaves or NIR laser light, the PANI NPs heat up and induce the phase transition of the thermosensitive hydrogel matrix and the internal solution is released. Other nano-objects, such as gold nanorods and PANI nanofibers, are also easily incorporated into the macroporous gel. The resulting nanocomposites also suffer a phase transition upon irradiation with electromagnetic waves. The results suggest that, using a thermosensitive matrix and conducting nanoparticles, mechanical/chemical actuators driven at a distance by electromagnetic radiation can be built. The sensitivity of the nanocomposite to electromagnetic radiation can be modulated by the pH, depending on the nature of the incorporated nanoparticles. Additionally, it is possible to make systems which absorb either NIR or microwaves or both.     

Stabilization of gold nanoparticle films on glass by thermal embedding

The poor adhesion of gold nanoparticles (NPs) to glass has been a known obstacle to studies and applications of NP-based systems, such as glass/Au-NP optical devices. Here we present a simple scheme for obtaining stable localized surface plasmon resonance (LSPR) transducers based on Au NP films immobilized on silanized glass and annealed. The procedure includes high-temperature annealing of the Au NP film, leading to partial embedding in the glass substrate and stabilization of the morphology and optical properties. The method is demonstrated using citrate-stabilized Au NPs, 20 and 63 nm mean diameter, immobilized electrostatically on glass microscope cover slides precoated with an aminosilane monolayer. Partial thermal embedding of the Au NPs in the glass occurs at temperatures in the vicinity of the glass transition temperature of the substrate. Upon annealing in air the Au NPs gradually settle into the glass and become encircled by a glass rim. In situ transmission UV-vis spectroscopy carried out during the annealing in a specially designed optical oven shows three regions: The most pronounced change of the surface plasmon (SP) band shape occurs in the first ca. 15 min of annealing; this is followed by a blue-shift of the SP band maximum (up to ca. 5 h), after which a steady red-shift of the SP band is observed (up to ca. 70 h, when the experiment was terminated). The development of the SP extinction spectrum was correlated to changes in the system structure, including thermal modification of the NP film morphology and embedding in the glass. The partially embedded Au NP films pass successfully the adhesive-tape test, while their morphology and optical response are stable toward immersion in solvents, drying, and thiol self-assembly. The enhanced adhesion is attributed to the metal NP embedding and rim formation. The stabilized NP films display a refractive index sensitivity (RIS) of 34-48 nm/RIU and 0.1-0.4 abs.u./RIU in SP band shift and extinction change, respectively. The RIS can be improved significantly by electroless deposition of Au on the embedded NPs, while the system stability is maintained. The method presented provides a simple route to obtaining stable Au NP film transducers.     

Self‐Assembled Large Au Nanoparticle Arrays with Regular Hot Spots for SERS

The cost‐effective self‐assembly of 80 nm Au nanoparticles (NPs) into large‐domain, hexagonally close‐packed arrays for high‐sensitivity and high‐fidelity surface‐enhanced Raman spectroscopy (SERS) is demonstrated. These arrays exhibit specific optical resonances due to strong interparticle coupling, which are well reproduced by finite‐difference time‐domain (FDTD) simulations. The gaps between NPs form a regular lattice of hot spots that enable a large amplification of both photoluminescence and Raman signals. At smaller wavelengths the hot spots are extended away from the minimum‐gap positions, which allows SERS of larger analytes that do not fit into small gaps. Using CdSe quantum dots (QDs) a 3–5 times larger photoluminescence enhancement than previously reported is experimentally demonstrated and an unambiguous estimate of the electromagnetic SERS enhancement factor of ≈10⁴ is obtained by direct scanning electron microscopy imaging of QDs responsible for the Raman signal. Much stronger enhancement of ≈10⁸ is obtained at larger wavelengths for benzenethiol molecules penetrating the NP gaps.     

Plasmonic focusing reduces ensemble linewidth of silver-coated gold nanorods

Silver coating gold nanorods reduces the ensemble plasmon line width by changing the relation connecting particle shape and plasmon resonance wavelength. This change, we term "plasmonic focusing", leads to less variation of resonance wavelengths for the same particle size distribution. We also find smaller single particle linewidth comparing resonances at the same wavelength but show that this does not contribute to the ensemble linewidth narrowing.     

Magnetic Plasmon Networks Programmed by Molecular Self‐Assembly

Nanoscale manipulation of magnetic fields has been a long‐term pursuit in plasmonics and metamaterials, as it can enable a range of appealing optical properties, such as high‐sensitivity circular dichroism, directional scattering, and low‐refractive‐index materials. Inspired by the natural magnetism of aromatic molecules, the cyclic ring cluster of plasmonic nanoparticles (NPs) has been suggested as a promising architecture with induced unnatural magnetism, especially at visible frequencies. However, it remains challenging to assemble plasmonic NPs into complex networks exhibiting strong visible magnetism. Here, a DNA‐origami‐based strategy is introduced to realize molecular self‐assembly of NPs forming complex magnetic architectures, exhibiting emergent properties including anti‐ferromagnetism, purely magnetic‐based Fano resonances, and magnetic surface plasmon polaritons. The basic building block, a gold NP (AuNP) ring consisting of six AuNP seeds, is arranged on a DNA origami frame with nanometer precision. The subsequent hierarchical assembly of the AuNP rings leads to the formation of higher‐order networks of clusters and polymeric chains. Strong emergent plasmonic properties are induced by in situ growth of silver upon the AuNP seeds. This work may facilitate the development of a tunable and scalable DNA‐based strategy for the assembly of optical magnetic circuitry, as well as plasmonic metamaterials with high fidelity.     

Biocompatible Conjugated Polymer Nanoparticles for Efficient Photothermal Tumor Therapy

Conjugated polymers (CPs) with strong near‐infrared (NIR) absorption and high heat conversion efficiency have emerged as a new generation of photothermal therapy (PTT) agents for cancer therapy. An efficient strategy to design NIR absorbing CPs with good water dispersibility is essential to achieve excellent therapeutic effect. In this work, poly[9,9‐bis(4‐(2‐ethylhexyl)phenyl)fluorene‐alt‐co‐6,7‐bis(4‐(hexyloxy)phenyl)‐4,9‐di(thiophen‐2‐yl)‐thiadiazoloquinoxaline] (PFTTQ) is synthesized through the combination of donor–acceptor moieties by Suzuki polymerization. PFTTQ nanoparticles (NPs) are fabricated through a precipitation approach using 1,2‐distearoyl‐ sn ‐glycero‐3‐phosphoethanolamine‐N‐[methoxy(polyethylene glycol)‐2000] (DSPE‐PEG₂₀₀₀) as the encapsulation matrix. Due to the large NIR absorption coefficient (3.6 L g^‐1 cm^‐1), the temperature of PFTTQ NP suspension (0.5 mg/mL) could be rapidly increased to more than 50 °C upon continuous 808 nm laser irradiation (0.75 W/cm²) for 5 min. The PFTTQ NPs show good biocompatibility to both MDA‐MB‐231 cells and Hela cells at 400 μg/mL of NPs, while upon laser irradiation, effective cancer cell killing is observed at a NP concentration of 50 μg/mL. Moreover, PFTTQ NPs could efficiently ablate tumor in in vivo study using a Hela tumor mouse model. Considering the large amount of NIR absorbing CPs available, the general encapsulation strategy will enable the development of more efficient PTT agents for cancer or tumor therapy.     

Metal nanoparticle plasmon-enhanced light-harvesting in a photosystem I thin film

Silver metal nanoparticle (NP) enhanced fluorescence is investigated in thin films of cyanobacterial Photosystem I trimer complexes (PSI) by correlating confocal laser scanning microscopy, dark-field imaging, and fluorescence lifetime measurements. PSI represents an interesting light-harvesting complex with a 20 nm diameter that is not uniformly contained within the surface-localized plasmon field of the NPs. With weak far-field illumination, 5- to 20-fold fluorescence enhancement is observed for PSI complexes adjacent to NPs, arising from efficient nanoparticle light collection and subsequent localized, surface plasmon excitation of PSI. Enhanced PSI fluorescence is detected most prominently near "rafts" of aggregated NPs that more completely fill the confocal field of view. These results demonstrate opportunities to probe energy transfer within photosynthetic complexes using plasmonic excitation and to design nanostructures for optimizing artificial light-harvesting systems.     

Sensitivity Tuning through Additive Heterogeneous Plasmon Coupling between 3D Assembled Plasmonic Nanoparticle and Nanocup Arrays

Plasmonic substrates have fixed sensitivity once the geometry of the structure is defined. In order to improve the sensitivity, significant research effort has been focused on designing new plasmonic structures, which involves high fabrication costs; however, a method is reported for improving sensitivity not by redesigning the structure but by simply assembling plasmonic nanoparticles (NPs) near the evanescent field of the underlying 3D plasmonic nanostructure. Here, a nanoscale Lycurgus cup array (nanoLCA) is employed as a base colorimetric plasmonic substrate and an assembly template. Compared to the nanoLCA, the NP assembled nanoLCA (NP‐nanoLCA) exhibits much higher sensitivity for both bulk refractive index sensing and biotin–streptavidin binding detection. The limit of detection of the NP‐nanoLCA is at least ten times smaller when detecting biotin–streptavidin conjugation. The numerical calculations confirm the importance of the additive plasmon coupling between the NPs and the nanoLCA for a denser and stronger electric field in the same 3D volumetric space. Tunable sensitivity is accomplished by controlling the number of NPs in each nanocup, or the number density of the hot spots. This simple yet scalable and cost‐effective method of using additive heterogeneous plasmon coupling effects will benefit various chemical, medical, and environmental plasmon‐based sensors.     

Fabrication of a Au/Si nanocomposite structure by nanosecond pulsed laser irradiation

A gold/silicon nanocomposite structure (NCS) was formed on a Si(100) surface by nanosecond pulsed laser irradiation. The Au/Si NCS contained both Au nanoparticles (NPs) and Au-Si alloy layers. We report that the use of laser irradiation to form Au NPs comprises two competing processes: a top-down effect involving decomposition into smaller NPs and a bottom-up effect involving self-assembly or self-organization into larger NPs. The formation of the periodic structure involved self-organization, i.e., the bottom-up effect, and was observed in situ using a pulsed-laser-equipped high-voltage electron microscope. The NCS formed by laser irradiation can be controlled by adjusting the laser energy density and the number of laser pulses.     

Nanorod-coated PNIPAM microgels: thermoresponsive optical properties

Nanoparticles and in particular gold nanorods have interesting optical properties arising from two well-differentiated plasmon modes. The frequency of such modes can be altered by their chemical environment and coupling with neighboring rods. This study investigates new composite materials made of gold nanorods adsorbed on thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) microgels. It is shown that the thermally induced collapse of the polymer network inside the particles leads to a red shift of the longitudinal plasmon band of the gold rods, which is found to be fully reversible.     

Synthesis and characterization of core∕shell Fe(3)O(4)∕ZnSe fluorescent magnetic nanoparticles

We report on the successful preparation and characterization of fluorescent magnetic core∕shell Fe(3)O(4)∕ZnSe nanoparticles (NPs) with a spherical shape by organometallic synthesis. The 7 nm core∕3 nm shell NPs show good magnetic and photoluminescence (PL) responses. The observed PL emission∕excitation spectra are shifted to shorter wavelengths, compared to a reference ZnSe NP sample. A dramatic reduction of PL quantum yield is also observed. The temperature dependence of the magnetization for the core∕shell NPs shows the characteristic features of two coexisting and interacting magnetic (Fe(3)O(4)) and nonmagnetic (ZnSe) phases. Compared to a reference Fe(3)O(4) NP sample, the room-temperature Néel relaxation time in core∕shell NPs is three times longer.