Two orders of magnitude fluorescence enhancement of aluminum phthalocyanines by gold nanocubes: a remarkable improvement for cancer cell imaging and detection

Flexible nanohybrid substrates utilizing gold nanocubes/nano mica platelets with 3D lightning-rod effect for highly efficient bacterial biosensors based on surface-enhanced Raman scattering

In this study, gold nanocubes (AuNCs) were quickly synthesized using the seed-mediated growth method and reduced onto the surface of two-dimensional (2D) delaminated nano mica platelets (NMPs), enabling the development of AuNCs/NMPs nanohybrids with a 3D lightning-rod effect. First, the growth-solution amount can be changed to easily adjust the AuNCs average-particle size within a range of 30-70 nm. The use of the cationic surfactant cetyltrimethylammonium chloride as a protective agent allowed the surface of AuNCs and nanohybrids to be positively charged. Positively charged nanohybrid surfaces presented a good adsorption effect for detecting molecules with negative charges on the surface. Additionally, the NMP surfaces were rich in ionic charges and provided a large specific surface area for stabilizing the growth of AuNCs. Delaminated AuNCs/NMPs nanohybrids can generate a 3D hotspot effect through self-assembly to enhance the Raman signal. Surface-enhanced Raman scattering (SERS) is highly sensitive in detecting adenine biomolecules. Its limit of detection (LOD) and Raman enhancement factor reached 10-9 M and 3.6 × 108, respectively. Excellent reproducibility was obtained owing to the relatively regular arrangement of AuNC particles, and the relative standard deviation (RSD) was 10.7%. Finally, the surface of NMPs was modified by adding the hydrophilic poly(oxyethylene)-diamine (POE2000) and amphiphilic PIB-POE-PIB copolymer at different weight ratios. The adjustment of the surface hydrophilicity and hydrophobicity of AuNCs/NMPs nanohybrids led to better adsorption and selectivity for bacteria. AuNCs/POE/NMPs and AuNCs/PIB-POE-PIB/NMPs were further applied to the SERS detection of hydrophilic Staphylococcus aureus and hydrophobic Escherichia coli, respectively. The SERS-detection results suggest that the LOD of hydrophilic Staphylococcus aureus and hydrophobic Escherichia coli reached 92 CFU mL-1 and 1.6 × 102 CFU mL-1, respectively. The AuNCs/POE/NMPs and AuNCs/PIB-POE-PIB/NMPs nanohybrids had different hydrophilic-hydrophobic affinities, which greatly improved the selectivity and sensitivity for detecting bacteria with different hydrophilicity and hydrophobicity. Therefore, fast, highly selective, and highly sensitive SERS biological-detection results were obtained.

Recent advances in plasmon-enhanced luminescence for biosensing and bioimaging

Plasmon-enhanced luminescence (PEL) is a unique photophysical phenomenon in which the interaction between luminescent moieties and metal nanostructures results in a marked luminescence enhancement. PEL offers several advantages and has been extensively used to design robust biosensing platforms for luminescence-based detection and diagnostics applications, as well as for the development of many efficient bioimaging platforms, enabling high-contrast non-invasive real-time optical imaging of biological tissues, cells, and organelles with high spatial and temporal resolution. This review summarizes recent progress in the development of various PEL-based biosensors and bioimaging platforms for diverse biological and biomedical applications. Specifically, we comprehensively assessed rationally designed PEL-based biosensors that can efficiently detect biomarkers (proteins and nucleic acids) in point-of-care tests, highlighting significant improvements in the sensing performance upon the integration of PEL. In addition to discussing the merits and demerits of recently developed PEL-based biosensors on substrates or in solutions, we include a brief discussion on integrating PEL-based biosensing platforms into microfluidic devices as a promising multi-responsive detection method. The review also presents comprehensive details about the recent advances in the development of various PEL-based multi-functional (passive targeting, active targeting, and stimuli-responsive) bioimaging probes, highlighting the scope of future improvements in devising robust PEL-based nanosystems to achieve more effective diagnostic and therapeutic insights by enabling imaging-guided therapy.

Integration of black phosphorus and hollow-core anti-resonant fiber enables two-order magnitude enhancement of sensitivity for bisphenol A detection

Hollow core anti-resonant fiber (HARF) has found a handful applications in optical communications, nonlinear optics and high power delivery. The intrinsic property of the fiber also renders it an ideal candidate for biosensing, which has not been explored intensively. Herein, we demonstrate an optical fiber sensing platform, taking advantages of the state-of-the-art HARF technology and superior physicochemical properties of 2D material black phosphorus, for ultra-sensitive detection of bisphenol A (BPA) in blood and environmental samples. The specially designed HARF can not only achieve broadband transmission of light, but also confine light in the low refractive-index liquid core, ensuring maximum overlap of light and liquid core. Modification of the inner surface of HARF with 2D black phosphorus nanoflakes functionalized with fluorescently labeled BPA-specific aptamer provides a smart sensing interface enabling highly selective detection of BPA via measuring the fluorescence. The limit of detection is 1.69pM, which is more than two orders of magnitude enhancement compared to the conventional plate assay. The proposed assay is not interfered with the BPA analogues BPB and BPS. The long optical path with tight optical confinement greatly enhances the analyte-light interaction and improves the sensitivity of the sensing platform. The proposed sensing platform can be further developed for versatile applications.

Multifunctionality of gold nanoparticles: Plausible and convincing properties

In a couple of decades, nanotechnology has become a trending area in science due to it covers all subject that combines diverse range of fields including but not limited to chemistry, physics and medicine. Various metal and metal oxide nanomaterials have been developed for wide range applications. However, the application of gold nanostructures and nanoparticles has been received more attention in various biomedical applications. The unique property of gold nanoparticles (AuNPs) is surface plasmon resonance (SPR) that determine the size, shape and stability. The wide surface area of AuNPs eases the proteins, peptides, oligonucleotides, and many other compounds to tether and enhance the biological activity of AuNPs. AuNPs have multifunctionality including antimicrobial, anticancer, drug and gene delivery, sensing applications and imaging. This state-of-the-art review is focused on the role of unique properties of AuNPs in multifunctionality and its various applications.

Green synthesis of gold nanoparticle/gelatin/protein nanogels with enhanced bioluminescence/biofluorescence

Here we report the green synthesis of gelatin/protein hybrid nanogels containing gold nanoparticles (AuNPs) that collectively exhibit metal-enhanced luminescence/fluorescence (MEL/MEF). The gelatin/protein nanogels, prepared by genipin cross-linking of preformed gelatin/protein polyion complexes (PICs), exhibited sizes ranging between 50 and 200 nm, depending on the weight ratio of gelatin and protein. These nanogels serve as reducing and stabilizing agents for the AuNPs, allowing for nucleation in a gel network that exhibits colloidal stability and MEL/MEF. AuNP/gelatin/HRP and AuNP/gelatin/LTF nanogels presented an ~11-fold enhancement of bioluminescence in an HRP-luminol system and a ~50-fold fluorescence enhancement when compared to free LTF in cell uptake experiments. These hybrid nanogels show promise for optically enhanced diagnosis and other therapeutic applications.

Fluorescence-enhanced covalent organic framework nanosystem for tumor imaging and photothermal therapy

Fluorescent dyes, as a key factor in fluorescence imaging, usually exhibit a low signal-to-noise ratio (SNR) due to the limited loading capacities of delivery systems (usually less than 10.0 wt%) and their uncontrolled release. Herein, we developed a type of pH-responsive nanoplatform (MnO2/ZnCOF@Au&BSA) based on a zinc porphyrin covalent organic framework (COF), in which the zinc porphyrin (ZnPor) loading rate is 22.5 wt%. At pH = 7.4, the interlinked ZnPor in the assembly state did not show a fluorescence signal ("off" state). Together with the pH-triggered disintegration of ZnCOF in tumor cells (pH = 5.5), the scattered ZnPor displayed an obvious fluorescence signal recovery ("on" state). Simultaneously, the shed BSA-coated gold nanoparticles ingeniously caused the fluorescence signal to be further amplified through the metal-enhanced fluorescence effect, which was about 3.0-fold higher in vivo than in the free ZnPor group. Combined with the excellent photothermal therapy effect by the nanoplatform itself with the tumor inhibition rate of 79.5%, this nanosystem effectively solves the problem of low loading capacities and imaging SNR by traditional delivery systems, and successfully develops the potential of COFs for fluorescence imaging, achieving the purpose of integration of diagnosis and treatment.

AlPcS-loaded gold nanobipyramids with high two-photon efficiency for photodynamic therapy in vivo

Recent years have witnessed significant progress in the field of two-photon-activated photodynamic therapy (TP-PDT). However, traditional photosensitizer (PS)-based TP-PDT remains a critical challenge in clinics due to its low two-photon absorption cross sections. Here, we propose that the therapeutic activity of the current photosensitizer, sulfonated Al-phthalocyanine (AlPcS), can be efficiently excited via plasmonic-resonance energy transfer from the two-photon excited gold nanobipyramids (GBPs) and further generates cytotoxic singlet oxygen for cancer eradication. GBPs possess large two-photon absorption cross sections, excellent photostability, and biocompatibility, which can be used for a high two-photon light-harvesting material in biomedical applications. We compared the in vitro and in vivo capabilities of AlPcS-loaded GBPs as a TP-PDT agent for theranostic applications by benchmarking them against those of the extensively studied gold nanospheres (GNS) and nanorods (GNR). Although all these Au nanostructures could cause enhanced PS two-photon excitation fluorescence and improved singlet oxygen generation capability via the plasmonic resonance-energy transfer process, GBP-AlPcS exhibited the highest two-photon efficiency for photodynamic therapy. Remarkably, in vivo experiment results clearly indicated that the GBP-AlPcS caused efficient suppression of tumor growth and minimal adverse effects on orthotopic A549 human lung tumor xenografts. The system presents great efficiency in improving the treatment depth and precision of traditional photodynamic therapy.

Plasmonic Nanoparticles Driven Enhanced Light Amplification in a Local 2D and 3D Self-Assembly

We present fluorescence and a random lasing enhancement effect due to the interaction between gold nanoparticles (AuNPs) and Rhodamine 6G (Rh6G) dye. Non-covalently bounded dyes in the proximity of nanoparticles are studied in three systems of varying dimensionality: from (i) three-dimensional freely distributed suspensions, through (ii) quasi-two-dimensional multilamellar liposomes, to (iii) solid two-dimensional thin layers. Liposomes facilitate the formation of stable AuNPs/Rh6G composition showing enhanced fluorescence, while solid thin films exhibit plasmon-assisted random lasing.

pH-Regulated Synthesis of Trypsin-Templated Copper Nanoclusters with Blue and Yellow Fluorescent Emission

In this article, a simple protocol to prepare water-soluble fluorescent copper nanoclusters (CuNCs) using trypsin as a stabilizer and hydrazine hydrate as a reducing agent was reported. It was found that the pH of the reaction solution was critical in determining the fluorescence of CuNCs. CuNCs with blue and yellow fluorescent emission were obtained under basic and acidic conditions, respectively. Although the detailed formation mechanisms of these CuNCs required further analysis, the synthetic route was promising for preparing different fluorescent metal NCs for applications. With good water solubility and excellent photostability, the yellow-emitting CuNCs could serve as a fluorescence probe for detection of Hg²⁺ based on the aggregation-induced quenching mechanism. The fluorescence quenching efficiency had fantastic linearity to Hg²⁺ concentrations in the range of 0.1-100 μM, with a limit of detection of 30 nM. Additionally, the yellow-emitting CuNCs exhibited negligible cytotoxicity and were successfully applied to bioimaging of HeLa cells.

Fluorescence guided photothermal/photodynamic ablation of tumours using pH-responsive chlorin e6-conjugated gold nanorods

Photothermal/photodynamic therapies (PTT/PDT) have been widely accepted as non-invasive therapeutic modalities to erase tumours. However, both therapies face the problem of precisely locating tumours and reducing their side effects. Herein, chlorin e6 conjugated gold nanorod, (Ce6-PEG-AuNR), a type of gold nanorod-photosensitizer conjugate, is designed as a kind of nano-therapeutic agent to simultaneously realize combined PTT/PDT. Compared to free Ce6, the fluorescence of Ce6 adhered to the conjugate is effectively quenched by the longitudinal surface plasmon resonance (LSPR) of in the Ce6-PEG-AuNR. However, the specific fluorescence of Ce6 can be recovered in tumour tissue when Ce6 is separated from the conjugate owing to the cleavage of hydrazone bond between Ce6 and PEG caused by intracellular acidic conditions in tumour tissue. Based on this effect, we can precisely locate tumours and further kill cancer cells by combined PTT/PDT. In addition, the combined therapy (PTT/PDT) function is more efficient in cancer treatment than that of PTT or PDT alone. Therefore, Ce6-PEG-AuNR can serve as a promising dual-modal phototherapeutic agent as well as a tumour-sensitive fluorescent probe to diagnose and treat cancer.

Noble metal nanoparticles in biosensors: recent studies and applications

The aim of this review is to cover advances in noble metal nanoparticle (MNP)-based biosensors and to outline the principles and main functions of MNPs in different classes of biosensors according to the transduction methods employed. The important biorecognition elements are enzymes, antibodies, aptamers, DNA sequences, and whole cells. The main readouts are electrochemical (amperometric and voltametric), optical (surface plasmon resonance, colorimetric, chemiluminescence, photoelectrochemical, etc.) and piezoelectric. MNPs have received attention for applications in biosensing due to their fascinating properties. These properties include a large surface area that enhances biorecognizers and receptor immobilization, good ability for reaction catalysis and electron transfer, and good biocompatibility. MNPs can be used alone and in combination with other classes of nanostructures. MNP-based sensors can lead to significant signal amplification, higher sensitivity, and great improvements in the detection and quantification of biomolecules and different ions. Some recent examples of biomolecular sensors using MNPs are given, and the effects of structure, shape, and other physical properties of noble MNPs and nanohybrids in biosensor performance are discussed.

Optofluidic chips with nanochannels for dynamic molecular detection using enhanced fluorescence

The fabrication of a novel optofluidic chip using nanochannels optimized for DNA-stretched molecules and optical detection by enhanced fluorescence is reported. The chips are composed of a series of microchannels that allow the transport of molecules in the femto-liter per second inside a fluid or gas. The nanochannels are surrounded by a photonic crystal structure to enhance the emission of fluorescent light from the molecules, which can travel along the nanochannel, allowing for enhanced optical detection of the molecules in motion. The photonic crystal structure provides an enhancement up to 2.5 times in the light emitted from fluorescent molecules inside the nanochannels which increases to around 250 when normalized to the area of the nanochannels emitting fluorescence. The results may help to the detection of fluorescent molecules (like marked-DNA) in series by speeding it and allowing the use of less sophisticated equipment.

Extraordinary enhancement of porphyrin photocurrent utilizing plasmonic silver arrays

We demonstrate up to ∼630-fold enhancement of the photocurrent from a porphyrin monolayer on a plasmonic Ag-array electrode showing plasmon absorption in the Q-band region relative to that on a planar Ag electrode. The photocurrent obtained by the Q-band excitation in the plasmonic electrodes even exceeded that obtained by the Soret-band excitation in a normal, nonplasmonic electrode.

Combination of photothermal and photodynamic inactivation of cancer cells through surface plasmon resonance of a gold nanoring

We demonstrate effective inactivation of oral cancer cells SAS through a combination of photothermal therapy (PTT) and photodynamic therapy (PDT) effects based on localized surface plasmon resonance (LSPR) around 1064 nm in wavelength of a Au nanoring (NRI) under femtosecond (fs) laser illumination. The PTT effect is caused by the LSPR-enhanced absorption of the Au NRI. The PDT effect is generated by linking the Au NRI with the photosensitizer of sulfonated aluminum phthalocyanines (AlPcS) for producing singlet oxygen through the LSPR-enhanced two-photon absorption (TPA) excitation of AlPcS. The laser threshold intensity for cancer cell inactivation with the applied Au NRI linked with AlPcS is significantly lower when compared to that with the Au NRI not linked with AlPcS. The comparison of inactivation threshold intensity between the cases of fs and continuous laser illuminations at the same wavelength and with the same average power confirms the crucial factor of TPA under fs laser illumination for producing the PDT effect.

Fluorescence chemosensors for hydrogen sulfide detection in biological systems

A comprehensive review of the development of H2S fluorescence-sensing strategies, including sensors based on chemical reactions and fluorescence resonance energy transfer (FRET), is presented. The advantages and disadvantages of fluorescence-sensing strategies are compared with those of traditional methods. Fluorescence chemosensors, especially those used in FRET sensing, are highly promising because of their low cost, technical simplicity, and their use in real-time sulfide imaging in living cells. Potential applications based on sulfate reduction to H2S, the relationship between sulfate-reducing bacteria activity and H2S yield, and real-time detection of sulfate-reducing bacteria activity using fluorescence sensors are described. The current challenges, such as low sensitivity and poor stability, are discussed.

Gold mining for PDT: Great expectations from tiny nanoparticles

Among many and various products, born by the modern nanotechnology, gold nanoparticles roused a special interest of biomedical researchers. Unique features of the nanoparticles allow to use them not only as effective transporters for therapeutic agents but also as basic components of nanocomposite preparations intended for targeted photodynamic and photothermal therapy of tumours. In the review, physical, chemical and biological properties of gold nanoparticles which can promote PDT efficiency of a designed nanocomposite, are briefly characterized, and promising trends in creation of gold-containing composite photosensitizers are analysed.

Laser beam controlled drug release from Ce6-gold nanorod composites in living cells: a FLIM study

A new method to image drug release from drug-nanoparticle composites in living cells was established. The composites of silica coated gold nanorods (AuNR@SiO2) and chlorine e6 (Ce6) photosensitizers (AuNR@SiO2-Ce6) were formed by electrostatic force with a Ce6 loading efficiency of 80%. The strong resonance absorptions of AuNR@SiO2-Ce6 in the near-infrared (NIR) region enabled the effective release of Ce6 from AuNR@SiO2-Ce6 by 780 nm CW laser irradiation. The 780 nm laser beam was applied to not only control the releasing amount of Ce6 from cellular AuNR@SiO2-Ce6 by adjusting the irradiation dose (time), but also to spatially confine the Ce6 release in cells by focusing the laser beam on the target sites. Furthermore, the fluorescence lifetime of Ce6 was found to change drastically from 0.9 ns in the AuNR@SiO2-Ce6 complex to 6 ns after release, and therefore fluorescence lifetime imaging microscopy (FLIM) was introduced to image the photo-induced Ce6 release in living cells. Finally, the controllable killing effect of photodynamic cancer therapy (PDT) using AuNR@SiO2-Ce6 was demonstrated by changing the released amount of Ce6, which indicates that AuNR@SiO2-Ce6 is promising for targeted tumour PDT.

Plasmon enhanced fluorescence of a bisphthalonitrile-based dye via a dopamine mediated interfacial crosslinking reaction on silver nanoparticles

Dopamine modified silver nanoparticles have been employed to enhance the fluorescence of a bisphthalonitrile end-capping phenolphthalein dye in solution via plasmon enhanced fluorescence methodology.