Nanoparticles chemosensors

Fig. 32 Effect of analyte coordination in (a) a traditional molecular chemosensor system, (b) receptors wired in series in a conjugated polymer, (c) chemosensors grafted onto the surface of a nanoparticle, and (d) a fluorescent dendrimer. The curved arrows indicate the active...

Bonacchi S, Rampazzo E, Montalti M et al (2008) Amplified fluorescence response of chemosensors grafted onto silica nanoparticles. Langmuir 24 8387-8392... 

PEBBLEs are water-soluble nanoparticles based on biologically inert matrices of cross-linked polymers, typically poly(acrylamide), poly(decylmethacrylate), silica, or organically modified silicates (ORMOSILs), which encapsulate a fluorescent chemo-sensor and, often, a reference dye. These matrices have been used to make sensors for pH, metal ions, as well as for some nonionic species. The small size of the PEBBLE sensors (from 20 to 600 nm) enables their noninvasive insertion into a living cell, minimizing physical interference. The semipermeable and transparent nature of the matrix allows the analyte to interact with the indicator dye that reports the interaction via a change in the emitted fluorescence. Moreover, when compared to naked chemosensors, nanoparticles can protect the indicator from chemical interferences and minimize its toxicity. Another important feature of PEBBLEs, particularly valuable in intracellular sensing applications, is that the polymer matrix creates a separate... 

Prodi L, Luminescent chemosensors From molecules to nanoparticles, New J. Chem., 2005, 29, 20-31. 

One of the most applicable metal nanoparticles is gold nanoparticles which have been used in different fields. These applications are enzymatic biosensors based on gold nanoparticles and their applications in genosensors, immunosen-sors, and electrocatalytic chemosensors (Fredy, 2008). 

Bonacchi S, Genovese D, Juris R et al (2010) Luminescent chemosensors based on silica nanoparticles. Top Curr Chem [this volume]... 

Fig. 2 Comparison between the signals arising from a molecular chemosensor and a nanoparticle-based one...

Luminescent silica nanoparticles are, in our opinion, the most promising and valuable of aU the species presented till now in this brief introduction they are potentially interesting for many applications like energy production and storage, catalysis and in particular sensing. After a description of the most common preparation methods in Sect. 2, we will present the state of the art for these particles as far as their application as chemosensors is concerned. 

The silica matrix has the capability to protect the active material segregated inside the nanoparticle from external chemicals. Large species cannot, in fact, permeate inside the nanoparticle, while small ones can but with a much reduced diffusion coefficient. This feature still allows, on one hand, the use of NPs as chemosensors for analytes of small dimensions (the dye interacts in its ground state), and on the other hand it decreases the possibility of undesired photoreactions (the excited state of the dye cannot undergo bimolecular reactions), thus increasing the photostability of the fluorophores inside the nanoparticle. The inclusion in this kind of matrix also helps to provide the active species with an almost constant environment in chemical terms. 

We will discuss hereafter many recent examples of chemosensors based only on luminescent silica nanoparticles but, even if this can appear to be a narrow field, the scenario is instead very wide. Therefore, with the aim of clarity, we have divided them in two main sections, one dealing with systems presenting the signalling units on the surface (dye coated silica nanoparticles, DCSNs) and the other with systems presenting it segregated inside the silica matrix (DDSNs). Moreover, for both... 

Following the synthetic strategy of the DCSNs, we have demonstrated the possibility to take advantage of the spatial organization and electronic communication between chromophoric units on the surface of silica nanoparticles for the development of a self-organized Zn(II) fluorescent chemosensor [116]. We used a triethoxysilane derivative of TSQ (6-methoxy-(8-p-toluenesulfonamido)quinoline) to realize a multichromophoric network on the surface of preformed silica nanoparticles. TSQ is a widely used fluorescent chemosensor able to bind Zn(II) ions with good selectivity. It is characterized by an off-on response due to an internal charge transfer (ICT) in the Zn(II)TSQ and Zn(II)(TSQ)2 complexes (Fig. 17). 

These phenomena, together with the enhanced affinity toward the substrate (the association constant increases of almost four orders of magnitude in the NPs), induced by the self-organized network on the surface of the nanoparticles, lead to a great increase in the sensitivity of the system, and provide interesting hints for the development of new fluorescent chemosensors. The same TSQ derivative was included in the silica matrix by Mancin and coworkers [25] that reported how its fluorescence was still sensitive to the presence of zinc ions but no amphfication effect could be observed, as we will discuss more in detail in Sect. 3.1.2. 

Fig. 21 Coated silica nanoparticles based self-organized fluorescence chemosensor. Adapted with permission from [113]...

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