Hollow micro-/nanostructures are of great interest in many current and emerging areas of technology. Perhaps the best-known example of the former is the use of fly-ash hollow particles generated from coal power plants as partial replacement for Portland cement, to produce concrete with enhanced strength and durability. This review is devoted to the progress made in the last decade in synthesis and applications of hollow micro-/nanostructures. We present a comprehensive overview of synthetic strategies for hollow structures. These strategies are broadly categorized into four themes, which include well-established approaches, such as conventional hard-templating and soft-templating methods, as well as newly emerging methods based on sacrificial templating and template-free synthesis. Success in each has inspired multiple variations that continue to drive the rapid evolution of the field. The Review therefore focuses on the fundamentals of each process, pointing out advantages and disadvantages where appropriate. Strategies for generating more complex hollow structures, such as rattle-type and nonspherical hollow structures, are also discussed. Applications of hollow structures in lithium batteries, catalysis and sensing, and biomedical applications are reviewed.

Hollow Micro-/Nanostructures: Synthesis and Applications**

Gold nanoparticles have been used in biomedical applications since their first colloidal syntheses more than three centuries ago. However, over the past two decades, their beautiful colors and unique electronic properties have also attracted tremendous attention due to their historical applications in art and ancient medicine and current applications in enhanced optoelectronics and photovoltaics. In spite of their modest alchemical beginnings, gold nanoparticles exhibit physical properties that are truly different from both small molecules and bulk materials, as well as from other nanoscale particles. Their unique combination of properties is just beginning to be fully realized in range of medical diagnostic and therapeutic applications. This critical review will provide insights into the design, synthesis, functionalization, and applications of these artificial molecules in biomedicine and discuss their tailored interactions with biological systems to achieve improved patient health. Further, we provide a survey of the rapidly expanding body of literature on this topic and argue that gold nanotechnology-enabled biomedicine is not simply an act of ‘gilding the (nanomedicinal) lily’, but that a new ‘Golden Age’ of biomedical nanotechnology is truly upon us. Moving forward, the most challenging nanoscience ahead of us will be to find new chemical and physical methods of functionalizing gold nanoparticles with compounds that can promote efficient binding, clearance, and biocompatibility and to assess their safety to other biological systems and their long-term term effects on human health and reproduction (472 references).

The golden age: gold nanoparticles for biomedicine

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

Functional Nanomaterials for Phototherapies of Cancer

Drug delivery, magnetic resonance and fluorescence imaging, magnetic manipulation, and cell targeting are simultaneously possible using a multifunctional mesoporous silica nanoparticle. Superparamagnetic iron oxide nanocrystals were encapsulated inside mesostructured silica spheres that were labeled with fluorescent dye molecules and coated with hydrophilic groups to prevent aggregation. Water-insoluble anticancer drugs were delivered into human cancer cells; surface conjugation with cancer-specific targeting agents increased the uptake into cancer cells relative to that in non-cancerous fibroblasts. The highly versatile multifunctional nanoparticles could potentially be used for simultaneous imaging and therapeutic applications.

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Multifunctional Inorganic Nanoparticles for Imaging, Targeting, and Drug Delivery

Gold nanocages with a relatively small size (e.g., ∼45 nm in edge length) have been developed, and the structure of these nanocages was tailored to achieve strong absorption in the near-infrared (NIR) region for photothermal cancer treatment. Numerical calculations show that the nanocage has a large absorption cross section of 3.48 × 10-14 m2, facilitating conversion of NIR irradiation into heat. The gold nanocages were conjugated with monoclonal antibodies (anti-HER2) to target epidermal growth factor receptors (EGFR) that are overexpressed on the surface of breast cancer cells (SK-BR-3). Our preliminary photothermal results show that the nanocages strongly absorb light in the NIR region with an intensity threshold of 1.5 W/cm2 to induce thermal destruction to the cancer cells. In the intensity range of 1.5−4.7 W/cm2, the circular area of damaged cells increased linearly with the irradiation power density. These results suggest that this new class of bioconjugated gold nanostructures, immuno gold nanocag...

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Immuno Gold Nanocages with Tailored Optical Properties for Targeted Photothermal Destruction of Cancer Cells

We report the ultrafast optical Kerr effect of Ag–BaO composite thin films by the femtosecond time-resolved pump-probe technique. The Ag–BaO thin films with Ag nanoparticles embedded into the BaO semiconductor matrix were prepared using a vacuum evaporation-deposition multimetallic layer method. The third-order nonlinear optical susceptibility of the thin films with the thickness of approximately 300 nm and the volume fraction of Ag nanoparticles in the thin films of about 25% was estimated to be 4.8×10−10 esu at the incident laser wavelength of 820 nm. The response time, i.e., the full width at half maximum of the Kerr signal, was as fast as 210 fs. The intrinsic third-order optical nonlinearity, or the optical Kerr effect of the thin films, can be attributed to the change of refractive index due to the intraband transition of electrons from the occupied state near the Fermi level to the unoccupied state in the Ag nanoparticles. Such nonlinearity is further enhanced by the local field effect that is present when the metallic nanoparticles are embedded into the semiconductor matrix.

Ultrafast optical Kerr effect of Ag-BaO composite thin films

Thin films of TiO2(B) nanowires are known to have sensitive and fast response to vapors of nitro-explosives under ambient conditions. The sensing response is believed to be affected by the humidity of ambient air that changes the density of hydroxyl groups on the TiO2(B) surfaces. To verify this mechanism, the role of surface hydroxyl groups on TiO2(B) nanowires was investigated via various surface modifications. It was found that a higher density of surface hydroxyl groups will constantly enhance the chemiresistive response of TiO2(B) nanowires to the nitro-explosives vapors. These surface hydroxyl groups serve as a pathway for effective charge transfer between the nitro groups on the explosive molecules and the TiO2(B). The evidence of charge transfer complex formation between nitro groups and titanium dioxide is also confirmed by Fourier transform infrared spectroscopy.

Sensitivity of titania(B) nanowires to nitroaromatic and nitroamino explosives at room temperature via surface hydroxyl groups

Environmental humidity is an important factor that can influence the sensing performance of a metal oxide. TiO2-(B) in the form of nanowires has been demonstrated to be a promising material for the detection of explosive gases such as 2,4,6-trinitrotoluene (TNT). However, the elimination of cross-sensitivity of the explosive detectors based on TiO2-(B) toward environmental humidity is still a major challenge. It was found that the cross-sensitivity could be effectively modulated when the thin film of TiO2-(B) nanowires was exposed to ultraviolet (UV) light during the detection of explosives under operating conditions. Such a modulation of sensing responses of TiO2-(B) nanowires to explosives by UV light was attributed to a photocatalytic effect, with which the water adsorbed on the TiO2-(B) nanowire surface was split and therefore the sensor response performance was less affected. It was revealed that the cross-sensitivity could be suppressed up to 51% when exposed to UV light of 365 nm wavelength with an intensity of 40 mW cm−2. This finding proves that the reduction of cross-sensitivity to humidity through UV irradiation is an effective approach that can improve the performance of a sensor based on TiO2-(B) nanowires for the detection of explosive gas.

Reducing cross-sensitivity of TiO2-(B) nanowires to humidity using ultraviolet illumination for trace explosive detection

Acetone existing in human breath is an effective biomarker of diabetes, which can be used for the early diagnosis and daily monitoring the health status of diabetes (in particular, type 1 diabetes). Comparing to other conventional methods for diabetes diagnosis and monitoring based on analyzing blood glucose level, breath analysis of acetone is a method with merits, such as non-invasive, accurate, convenient, and inexpensive. A novel nanostructured K2W7O22 was recently developed and tested on its feasibility for acetone detection. The results show that K2W7O22 can effectively detect a trace amount of acetone at room temperature. The lowest detection limit ~2.0 ppm with a fast response time of 12 s is achieved. Further improvement of sensing performance with detection limit down to 25 ppb of acetone can be achieved through optimizing the material properties of K2W7O22 and modifying the design of device circuit to realize weak signal detection. The excellent acetone response is studied due to the unique properties of K2W7O22, ferroelectric and semiconducting, which result in the effective interaction and strong charge transfer between acetone and K2W7O22. This paper can improve the understanding of the new material and its sensing mechanism and thus give ideas for further increasing the sensitivity for acetone detection, eventually resulting in an advanced material capable of analyzing acetone in exhaled breath for disease diagnosis and monitoring purpose.

Danling Wang

Papers

Immuno Gold Nanocages with Tailored Optical Properties for Targeted Photothermal Destruction of Cancer Cells

Ultrafast optical Kerr effect of Ag-BaO composite thin films

Sensitivity of titania(B) nanowires to nitroaromatic and nitroamino explosives at room temperature via surface hydroxyl groups

Reducing cross-sensitivity of TiO2-(B) nanowires to humidity using ultraviolet illumination for trace explosive detection

High Sensitive Breath Sensor Based on Nanostructured K 2 W 7 O 22 for Detection of Type 1 Diabetes