Luca Guerrini

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications.

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.

Sensing polycyclic aromatic hydrocarbons with dithiocarbamate-functionalized ag nanoparticles by surface-enhanced Raman scattering.

Trace detection of polycyclic aromatic hydrocarbons is reported in this work on dithiocarbamate calix[4]arene functionalized Ag nanoparticles by using surface-enhanced Raman scattering (SERS). SERS spectra informed about the existence of the pollutant by measuring its characteristic fingerprint vibrational features. In addition, SERS revealed important structural information from both the host and the analyte which was crucial to understand and deduce the host-guest interaction mechanism. The effectiveness of this system was checked for a group of PAHs: pyrene, benzo[c]phenanthrene, triphenylene, and coronene. From the analyzed results, the affinity constants and the limit of detection were deduced for each pollutant.

Nanosensors based on viologen functionalized silver nanoparticles: few molecules surface-enhanced Raman spectroscopy detection of polycyclic aromatic hydrocarbons in interparticle hot spots.

The functionalization of silver nanoparticles (Ag NPs) by viologen dications (VGDs) is reported in this work as well as their applications in the surface-enhanced Raman scattering (SERS) detection of polycyclic aromatic hydrocarbons (PAHs). VGDs are able to form intermolecular cavities at interparticle junctions (SERS hot spots) where the analyte can be allocated. This leads to a giant intensification of the Raman emission of the target molecule. This effect was applied in the detection of PAHs, one of the most widespread and dangerous group of pollutants existing in the atmosphere and waters. A comparison between sensing-systems based on different VGDs (lucigenin, diquat, and paraquat) was done for the detection of two PAHs (pyrene and benzo[c]phenanthrene). The functionalization with lucigenin (LG) provided the most powerful and stable VGD-NPs sensor, which allowed the SERS detection of pyrene (PYR) down to 10(-9) M in the macro setup and in the zeptomole range for spectra obtained by single NPs aggregates (micro setup). Besides, SERS spectra afforded important structural information about the interaction mechanism of VGD and PAHs, revealing the formation of a CT complex between the VGD and PYR and changes in the host conformation. The position of the nu(Ag-Cl) band and the plasmon resonance contribution assigned to Ag dimers were also used as spectral markers to monitor the host-guest interaction.

Importance of Nanoparticle Size in Colorimetric and SERS-Based Multimodal Trace Detection of Ni(II) Ions with Functional Gold Nanoparticles

Colorimetric detection of analytes using gold nanoparticles along with surface-enhanced Raman spectroscopy (SERS) are areas of intense research activity since they both offer sensing of very low concentrations of target species. Multimodal detection promotes the simultaneous detection of a sample by a combination of different techniques; consequently, surface chemistry design in the development of multimodal nanosensors is important for rapid and sensitive evaluation of the analytes by diverse analytical methods. Herein it is shown that nanoparticle size plays an important role in the design of functional nanoparticles for colorimetric and SERS-based sensing applications, allowing controlled nanoparticle assembly and tunable sensor response. The design and preparation of robust nanoparticle systems and their assembly is reported for trace detection of Ni(II) ions as a model system in an aqueous solution. The combination of covalently attached nitrilotriacetic acid moieties along with the L-carnosine dipeptide on the nanoparticle surface represents a highly sensitive platform for rapid and selective detection of Ni(II) ions. This systematic study demonstrates that significantly lower detection limits can be achieved by finely tuning the assembly of gold nanoparticles of different core sizes. The results clearly demonstrate the feasibility and usefulness of a multimodal approach.

Highly Sensitive SERS Quantification of the Oncogenic Protein c‑Jun in Cellular Extracts

A surface-enhanced Raman scattering (SERS)-based sensor was developed for the detection of the oncoprotein c-Jun at nanomolar levels. c-Jun is a member of the bZIP (basic zipper) family of dimeric transcriptional activators, and its overexpression has been associated with carcinogenic mechanisms in several human cancers. For our sensing purpose, we exploited the ability of c-Jun to heterodimerize with its native protein partner, c-Fos, and therefore designed a c-Fos peptide receptor chemically modified to incorporate a thiophenol (TP) group at the N-terminal site. The TP functionality anchors the c-Fos protein onto the metal substrate and works as an effective SERS probe to sense the structural rearrangements associated with the c-Fos/c-Jun heterodimerization.

Direct Surface-Enhanced Raman Scattering Analysis of DNA Duplexes†

The exploration of the genetic information carried by DNA has become a major scientific challenge. Routine DNA analysis, such as PCR, still suffers from important intrinsic limitations. Surface-enhanced Raman spectroscopy (SERS) has emerged as an outstanding opportunity for the development of DNA analysis, but its application to duplexes (dsDNA) has been largely hampered by reproducibility and/or sensitivity issues. A simple strategy is presented to perform ultrasensitive direct label-free analysis of unmodified dsDNA with the means of SERS by using positively charged silver colloids. Electrostatic adhesion of DNA promotes nanoparticle aggregation into stable clusters yielding intense and reproducible SERS spectra at nanogram level. As potential applications, we report the quantitative recognition of hybridization events as well as the first examples of SERS recognition of single base mismatches and base methylations (5-methylated cytosine and N6-methylated Adenine) in duplexes.

Universal One-Pot and Scalable Synthesis of SERS Encoded Nanoparticles

Encoded particles are one of the most powerful approaches for multiplex high-throughput screening. Surfaceenhanced Raman scattering (SERS) based codification can, in principle, avoid many of the intrinsic limitations due to conventional alternatives, as it decreases the reading time and particle size while allowing for almost unlimited codification. Unfortunately, methods for the synthetic preparation of these particles are tedious; often subjected to limited reproducibility (associated with large fluctuations in the size distributions of the polymers employed in the standard protocols); and to date, limited to a small amount of molecules. Herein, we report a universal, one-pot, inexpensive, and scalable synthetic protocol for the fabrication of SERS-encoded nanoparticles. This synthetic strategy is highly reproducible, independent of the chemical nature and size of the Raman code used (31 different codes were tested) and scalable in the liter range without affecting the final properties of the encoded structures. Furthermore, the SERS efficiency of the fabricated encoded nanoparticles is superior to that of the materials produced by conventional methods, while showing a remarkable reproducibility from batch to batch. This encoding strategy can easily be applied to nanoparticles of different materials and shapes.

Directed assembly of DNA-functionalized gold nanoparticles using pyrrole-imidazole polyamides.

Traditional methods for the construction of nanoparticle arrays and lattices exploit Watson-Crick base pairing of single-stranded DNA sequences as a proxy for self-assembly. Although this approach has been utilized in a variety of applications in nanoassembly, diagnostics, and biomedicine, the diversity of this recognition lexicon could be considerably increased by developing strategies that recognize the base-pairing landscape of double-stranded DNA (dsDNA) sequences. Herein we describe the first report of programmed gold nanoparticle (GNP) aggregation directed by the recognition of dsDNA sequences using pyrrole-imidazole polyamide-GNP (PA-GNP) conjugates. We demonstrate the reversibility and selectivity of this strategy for forming GNP aggregates in the presence of fully matched dsDNA sequences relative to dsDNA sequences containing one- and two-base-pair mismatches.

Functionalization of Ag nanoparticles with the bis-acridinium lucigenin as a chemical assembler in the detection of persistent organic pollutants by surface-enhanced Raman scattering.

Organochlorine pesticide endosulfan has been detected for the first time by using surface-enhanced Raman scattering (SERS) at trace concentrations. The bis-acridinium dication lucigenine was successfully used as a molecular assembler in the functionalization of metal nanoparticles to facilitate the approach of the pesticide to the metal surface. From the SERS spectra valuable information about the interaction mechanism between the pesticide and lucigenin can be deduced. In fact, endosulfan undergoes an isomerization upon adsorption onto the metal, while the viologen undergoes a rotation of the acridinium planes to better accommodate the pesticide molecule. An interaction between the N atom of the central acridinium ring and the pesticide Cl-CC-Cl fragment is verified through a charge-transfer complex. The present study affords important information which can be applied to the design of chemical sensor systems of persistent organic pollutants based on the optical detection on functionalized metal nanoparticle.

Tuning the interparticle distance in nanoparticle assemblies in suspension via DNA-triplex formation : correlation between plasmonic and surface-enhanced Raman scattering responses

The understanding of the relationship between plasmonic and surface-enhanced Raman scattering (SERS) properties of dynamic nanoparticle assemblies is of paramount importance for the optimal design of related plasmonic nanostructures, especially for SERS applications. In this regard, recent studies have provided new important insights for well-ordered nanoparticle assemblies but little is known about the relationship between the physical and optical properties for large ensembles of randomly aggregated metal nanoparticles in suspension, which still represents the simplest and most common route to obtain highly effective SERS substrates. Here we exploit the triplex-assembling ability of DNA-conjugated silver nanoparticles to engineer interparticle junctions with controlled interparticle distance and tune the aggregation rate to allow accurate investigation into the correlation between the averaged time-dependent plasmonic and SERS responses within a complex ensemble of nanoparticles in suspension. Solution-based single particle tracking was used to characterize the heterogeneity of the nanoparticle assembly with statistical reliability, acting as a key tool to unravel the connection between these two bulk responses. To achieve this, we report the first example of the parallel hybridization of dye-labeled locked nucleic acid (LNA) silver nanoparticle probes to double stranded DNA bridges of different lengths to form a triplex assembly, that provides SERS enhancements directly related to the interparticle distance imposed by the high structural rigidity of the double stranded linker. This is also a crucial step towards utilising SERS for the study of DNA in its natural double stranded state and, ultimately, to obtain nanoscale distance-dependent information in challenging biological environments using specially designed nanoparticles.