Tuning the Sensing Performance of Multilayer Plasmonic Core-Satellite Assemblies for Rapid Detection of Targets from Lysed Cells

Core-satellite assembly of gold nanoshells on solid gold nanoparticles for a color coding plasmonic nanosensor

We present core-satellite assemblies comprising a solid gold nanoparticle as the core and hollow decahedral gold nanoshells as satellites for tuning the optical properties of the plasmonic structure for sensing. The core-satellite assemblies were fabricated on a substrate via the layer-by-layer assembly of nanoparticles linked by DNA. We used finite-difference time-domain simulations to help guide the geometrical design, and characterized the optical properties and morphology of the solid-shell nanoparticle assemblies using darkfield microscopy, single-nanostructure spectroscopy, and scanning electron microscopy. Plasmon coupling yielded resonant peaks at longer wavelengths in the red to near-infrared range for solid-shell assemblies compared with solid-solid nanoparticle assemblies. We examined sensing with the solid-shell assemblies using adenosine triphosphate (ATP) as a model target and ATP-aptamer as the linker. Binding of ATP induced disassembly and led to a decrease in the scattering intensity and a color change from red to green. The new morphology of the core-satellite assembly enabled plasmonic color-coding of multiplexed sensors. We demonstrate this potential by fabricating two types of assemblies using DNA linkers that target different molecules - ATP and a model nucleic acid. Our work expands the capability of chip-based plasmonic nanoparticle assemblies for the analysis of multiple, different types of biomolecules in small sample sizes including the microenvironment and single cells.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

Algorithm-Assisted Detection and Imaging of microRNAs in Living Cancer Cells via the Disassembly of Plasmonic Core-Satellite Probes Coupled with Strand Displacement Amplification

Acute detection and high-resolution imaging of microRNAs (miRNAs) in living cancer cells have attracted great attention in clinical diagnosis and therapy. However, current methods suffer from low detection sensitivity or heavy dependence on expensive and sophisticated spectrometers. Herein, a novel algorithm-assisted system of detecting and imaging miRNAs in living cancer cells was developed via the disassembly of plasmonic core-satellite probes coupled with strand displacement amplification (SDA). The target miRNAs in the system could trigger the disassembly of plasmonic core-satellite probes, leading to the color change in the scattering light of the probes, which could be captured by dark-field microscopy (DFM). The concentration of the target miRNAs was obtained by analyzing the dark-field image based on the proposed algorithm with a detection limit of 2 pM for miRNA-21. Thus, the performance in terms of simplicity and sensitivity of the system compared with one of the conventional spectrophotometers was well presented, which could inspire more clinical applications of inexpensive, intelligent, and rapid screening of cancer cells. The application software based on the proposed algorithm running on the Android platform was also developed, demonstrating the potential of remote diagnosis.

Multiplexed and single-cell detection of microRNA with plasmonic nanoparticle assemblies

We present a label-free, low cost, and miniatured biosensing platform based on the disassembly of core-satellite plasmonic nanoparticle assemblies. The rapid and selective detection of an exemplary nucleic acid biomarker, has-miRNA-210-3p, was achieved via the strand displacement nucleic acid reaction. Target binding leads to dehybridization of the DNA linkers and changes in the scattering properties of nanostructures as monitored by darkfield microscopy. We demonstrate the ability to detect microRNA expunged from single cells and the potential to multiplex discrete assemblies to enable diverse biological applicability. The work may help translate the applicability of microRNA as diagnostic biomarkers, quantitate their abundance in the microenvironment, and facilitate the study of their correlation or causation to other biomolecules at the single-cell level.

Plasmonic gold nanostructures for biosensing and bioimaging

As one kind of noble metal nanostructures, the plasmonic gold nanostructures possess unique optical properties as well as good biocompatibility, satisfactory stability, and multiplex functionality. These distinctive advantages make the plasmonic gold nanostructures an ideal medium in developing methods for biosensing and bioimaging. In this review, the optical properties of the plasmonic gold nanostructures were firstly introduced, and then biosensing in vitro based on localized surface plasmon resonance, Rayleigh scattering, surface-enhanced fluorescence, and Raman scattering were summarized. Subsequently, application of the plasmonic gold nanostructures for in vivo bioimaging based on scattering, photothermal, and photoacoustic techniques  has been also briefly covered. At last, conclusions of the selected examples are presented and an outlook of this research topic is given.

Sensing Biomarkers with Plasmonics

The detection of biomarkers is critical for enabling early disease diagnosis, monitoring the progression, and tracking the effectiveness of therapeutic intervention. Plasmonic sensors exhibit a broad range of analytical capabilities, from the rapid generation of colorimetric readouts to single-molecule sensitivity in ultralow sample volumes, which have led to their increased exploration in bioanalysis and point-of-care applications. This perspective presents selected accounts of recent developments on the different types of plasmonic sensing platforms, the pervasive challenges, and outlook on the pathway to translation. We highlight the sensing of upcoming biomarkers, including microRNA, circulating tumor cells, exosomes, and cell-free DNA, and discuss the opportunity of utilizing plasmonic nanomaterials and tools for biomarker detection beyond biofluids, such as in tissues, organs, and disease sites. The integration of plasmonic biosensors with established and upcoming technologies of instrumentation, sample pretreatment, and data analysis will help realize their translation to clinical settings for improving healthcare and enhancing the quality of life.

Metabolic mapping with plasmonic nanoparticle assemblies

A rapid and simple methodology for the biomolecular analysis of single cells and microenvironments via a stick-and-peel plasmonic sensing platform is reported. Substrate-bound assemblies of plasmonic gold nanoparticles linked by reconfigurable oligonucleotides undergo disassembly upon target binding. Changes in the light scattering intensity of thousands of discrete nanoparticle assemblies are extrapolated concomitantly to yield the mapping of local target concentrations. The methodology is completely free of labelling, purification and separation steps. We quantified the intracellular ATP levels for two ovarian cancer cell lines to elucidate the differences and cellular distribution, and demonstrated the potential of the stick-and-peel platform for mapping the microenvironment of a 2D heterogeneous surface. The portable and economical analytical platform may broaden the affordability and applicability of single-cell based analyses and enable new opportunities in clinical care such as on-site molecular pathology.

Hybrid Silver Nanocubes for Improved Plasmon-Enhanced Singlet Oxygen Production and Inactivation of Bacteria

Plasmonic nanoparticles can strongly interact with adjacent photosensitizer molecules, resulting in a significant alteration of their singlet oxygen (¹O₂) production. In this work, we report the next generation of metal-enhanced ¹O₂ nanoplatforms exploiting the lightning rod effect, or plasmon hot spots, in anisotropic (nonspherical) metal nanoparticles. We describe the synthesis of Rose Bengal-decorated silica-coated silver nanocubes (Ag@SiO₂-RB NCs) with silica shell thicknesses ranging from 5 to 50 nm based on an optimized protocol yielding highly homogeneous Ag NCs. Steady-state and time-resolved ¹O₂ measurements demonstrate not only the silica shell thickness dependence on the metal-enhanced ¹O₂ production phenomenon but also the superiority of this next generation of nanoplatforms. A maximum enhancement of ¹O₂ of approximately 12-fold is observed with a 10 nm silica shell, which is among the largest ¹O₂ production metal enhancement factors ever reported for a colloidal suspension of nanoparticles. Finally, the Ag@SiO₂-RB NCs were benchmarked against the Ag@SiO₂-RB nanospheres previously reported by our group, and the superior ¹O₂ production of Ag@SiO₂-RB NCs resulted in improved antimicrobial activities in photodynamic inactivation experiments using both Gram-positive and -negative bacteria model strains.