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Farinha, J.P.S. Bright and Stable Nanomaterials for Imaging and Sensing. Encyclopedia. Available online: https://encyclopedia.pub/entry/52918 (accessed on 21 May 2024).
Farinha JPS. Bright and Stable Nanomaterials for Imaging and Sensing. Encyclopedia. Available at: https://encyclopedia.pub/entry/52918. Accessed May 21, 2024.
Farinha, José Paulo Sequeira. "Bright and Stable Nanomaterials for Imaging and Sensing" Encyclopedia, https://encyclopedia.pub/entry/52918 (accessed May 21, 2024).
Farinha, J.P.S. (2023, December 19). Bright and Stable Nanomaterials for Imaging and Sensing. In Encyclopedia. https://encyclopedia.pub/entry/52918
Farinha, José Paulo Sequeira. "Bright and Stable Nanomaterials for Imaging and Sensing." Encyclopedia. Web. 19 December, 2023.
Bright and Stable Nanomaterials for Imaging and Sensing
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This entry is adapted from the peer-reviewed paper 10.3390/polym15193935

Optical techniques for imaging and diagnosis are at the center of the wonderous developments in the biomedical field, which are paving the way to personalized medicine. Fluorescence-based techniques, in particular, have allowed dramatic progress in the analysis of biological systems and other complex processes. This is because they present better specificity, sensitivity, contrast, and versatility than techniques based on the absorption/reflection of light. Their performance is closely related to the brightness and photostability of the light-emitting materials: the amount of light emitted for a given illumination and the ability to withstand high-power illumination without degrading.

bright polymer nanomaterials fluorescence microscopy imaging fluorescence sensing

1. Polymer Chains with Multiple Fluorescent Dyes

Polymer chains of low molecular weight dispersity, well-defined architecture, and a controlled number of dyes provide extremely well-controlled vehicles that can cover a size range of only a few nanometers (well below the minimum size achievable by polymer nanoparticles). These vehicles are particularly promising owing to their versatility in terms of the monomers and mixtures of monomers that can be used and their modification with different groups (fluorescent, targeting groups, etc.). Polymer chains can be prepared with a very narrow size distribution and precisely controlled architecture and composition by using synthetic approaches based on controlled radical polymerization techniques [1]. To obtain a fluorescent polymer chain reporter with a homogeneous signal per reporter unit, one should covalently attach a large and known number of fluorescent dyes to the polymer chain so that they do not interact with one another (to avoid quenching of the emission).

2. Polymer Nanoparticles

An alternative approach is to use polymer nanoparticles (PNPs) to incorporate the dyes. Although nanoparticles from conjugated polymers are intrinsically fluorescent and very bright [2][3][4][5], they have limited application because of poor biodegradability and cumbersome stabilization. On the other hand, nanoparticles of non-fluorescent polymers or copolymers can be used to encapsulate fluorescent dyes, offering huge versatility due to the large number of available monomers, the possibility to mix different monomers, and the incorporation of different groups in the polymer. Additionally, there are a large number of different preparation procedures, and the polymers can be obtained in a wide range of sizes. The incorporation of fluorescent dyes in these nanoparticles can rely solely on the encapsulation of the molecules (a simple approach that can lead to the leaching of the dyes from the nanoparticles) or on the modification of monomers with fluorescent groups (which prevents dye leaching but involves more complex synthetic procedures).
Dye-loaded polymer nanoparticles can be obtained from preformed polymer chains by a number of different approaches, but these are usually associated with poor control over the size of the nanoparticles, their stability, and the distribution of the dyes in the nanoparticles. Emulsification of preformed polymers involves previously dissolving the polymer (and the dyes) in a water-immiscible, low boiling point solvent that is dispersed in water using a surfactant and strong shearing (mechanical or ultrasound) to yield nanoparticles with diameters in the hundreds of nanometers range [6]. In nanoprecipitation, the polymer with the dyes is dissolved in a water-miscible solvent, which, when added to water, leads to the (kinetically controlled) formation of nanoparticles in a wide range of diameters [7]. In one example, charge-controlled nanoprecipitation was proposed to improve the morphology control of the nanoparticles and the dye encapsulation using charged polymers and a salt of rhodamine B octadecyl ester (R18) with tetrakis(pentafluorophenyl)borate (F5-TPB) as a counterion [8].
Self-assembly of amphiphilic copolymers can also lead to nanoparticles (copolymer micelles), but since the process is generally thermodynamically driven, the nanoparticle stability can be drastically decreased at low concentrations, and the system must be carefully considered for different media [9]. The three methods can be adapted to use polymers previously labeled with covalently attached fluorescent dyes; however, since they rely on differences in solvation, the impact of the presence of the dyes in the polymer must be evaluated on an individual basis [10].
An approach offering better control over particle morphology is to obtain the nanoparticles directly by polymerizing the monomers. There are three major techniques for this, which involve different particle formation mechanisms: emulsion, miniemulsion, and microemulsion polymerization.
In the preparation of fluorescent polymer nanoparticles by emulsion polymerization, the monomers are emulsified in water, usually with one or more surfactants, with the initial system containing micron-size monomer droplets and surfactant micelles. The polymerization starts in the surfactant micelles due to their larger overall surface area, and the droplets act as monomer reservoirs to feed the particle growth in the micelles. Due to this mechanism, emulsion polymerization is very efficient in producing nanoparticles with extremely narrow size distributions and diameters ranging from ca. 30 nm to hundreds of nanometers. However, dye encapsulation is difficult because hydrophobic dyes (low solubility in water) tend to stay in the monomer droplets instead of migrating to the micelles, from where the particles grow. On the other hand, water-soluble dyes stay in the water phase, also leading to poor incorporation. This limitation can be circumvented by using fluorescent dyes modified with polymerizable groups [11][12] and incorporating them into the polymer chains using a starved feed approach where the monomer mixture is fed to the reactor at a controlled rate [13][14]. This allows the preparation of fluorescent nanoparticles with very homogeneous dye distribution and no aggregation or dye leaching for demanding applications requiring precise control of the nanoparticle size, morphology, and optical properties [15]. The disadvantages of this approach to producing bright fluorescent nanoparticles are that the particle size is relatively large (leading to light scattering when in dispersion), and it is difficult to reach very high dye content. Nevertheless, this approach has been extremely successful in preparing PNPs for characterizing polymer interphases by Forster resonance energy transfer (FRET), for example in waterborne polymer coatings prepared from mixtures of PNPs labeled with donor and acceptor dyes [15].
Miniemulsion polymerization offers lower control over particle size than emulsion polymerization but is much better suited for the encapsulation of hydrophobic molecules such as fluorescent dyes. The nanoparticles obtained by miniemulsion polymerization can be smaller than those obtained by emulsion polymerization (ca. 30 to 200 nm). In miniemulsion polymerizations, the droplet size is reduced by applying high shear forces (mechanical stirring or sonication), with the droplets stabilized by a surfactant and a water-insoluble co-stabilizer (e.g., hexadecane) [16]. The polymerization occurs inside the monomer droplets, which directly originate the nanoparticles. Therefore, dyes or other hydrophobic molecules initially present in the monomer droplets become encapsulated in the resulting nanoparticles. Miniemulsion polymerization is especially useful for the encapsulation of dyes that cannot be functionalized without affecting their optical properties, as in the case of fullerenes. In one example, C70 was successfully incorporated into polystyrene nanoparticles (PS) by miniemulsion polymerization so that the nanoparticles could be incorporated into different materials where C70 is not dispersible.
Microemulsion polymerization relies on thermodynamically stable emulsions of very small droplets stabilized with high amounts of surfactants [17], allowing the preparation of very small nanoparticles (from ca. 5 nm). The use of thermodynamically stable emulsions strongly limits the diversity of monomers that can be used in microemulsion and the application of this technique. One other disadvantage of microemulsion polymerization for preparing dye-loaded nanoparticles is that the dye distribution is not homogeneous among particles due to inhomogeneous nucleation (occurring in both droplets and in micelles). In this approach, the dyes can also be copolymerized [18] or incorporated after particle formation by swelling the nanoparticles [19].

3. Silica Nanoparticles

Silica is a highly cross-linked polymer obtained by the condensation polymerization of silicon-containing monomers, usually alkoxysilanes, which are hydrolyzed during the condensation process. Silica nanoparticles (SNPs) are widely used in different imaging applications, from optical techniques to magnetic resonance, positron emission, X-ray tomography, and ultrasound, as well as combinations of different techniques [20][21]. SNPs can be prepared by simple, scalable, and low-cost techniques and offer tunable and very well-defined size, morphology, and porosity. One of the most commonly used preparation methods was developed by Stobër, consisting of the base-catalyzed hydrolysis of the silica precursors, followed by condensation polymerization [22]. Precursors such as tetraethyl orthosilicate (TEOS) and other molecules containing alkoxysilane groups are used.
SNPs feature high chemical and mechanical stability, with the ability to protect guest molecules that are incorporated in the silica structure [23] or entrapped within [24]. Contrary to most PNPs, both the structure and the surface of SNPs can be easily functionalized during and post-synthesis, by well-established siloxane chemistry using organic alkoxysilane compounds for covalent immobilization of different groups. This strategy can be used to improve colloidal stability in different media or to incorporate polymers or (bio)molecules for biological targeting, to modulate interactions, for sensing, to control cargo release, etc.
Silica is endogenous to most living organisms, and SNPs are mostly biocompatible, having been long approved for human clinical trials [25]. For biomedical applications, SiNPs of spherical or near-spherical shape and diameters around or under 100 nm are preferred because they are easily internalized in cells, virtually nontoxic, and easily excreted from living organisms. However, in biological environments, bare SNPs are readily coated with proteins and attacked by the immune system, which can impact their performance. To avoid this, they can be surface-modified with polymers or biomolecules that provide stealth and adhesion control properties, with poly(ethylene oxide) being the most common option [26], although other strategies produce even more promising results [27].
SNPs offer excellent support for optical imaging applications since they are transparent in a wide range of wavelengths, from the ultraviolet to the near infrared (NIR) and can incorporate a large number of fluorophores. Fluorescent SNPs can be prepared with very well-defined particle morphology, with diameters ranging from ca. 10 nm to several hundred nanometers [28][29]. They can be doped with fluorophores, either by physically entrapping the dyes inside the particles, by coating the nanoparticle surface, or by covalently attaching the dyes modified with alkoxysilane groups to the silica network [28][30][31][32][33][34]. In all cases, attention has to be paid to the possible aggregation of the dyes that can lead to self-quenching [35][36].
An important advantage of incorporating dyes in SNPs is that the low oxygen diffusivity inside SNPs shields the dyes from oxygen, thus enhancing their photostability (this is especially relevant when using high-power excitation, for example, in laser scanning techniques) [28][37]. Additionally, dye-containing SNPs can be used in solvents where the free dyes are insoluble. Bare dye-loaded fluorescent SNPs are readily dispersed in water, allowing the use of water-insoluble dyes for imaging applications in aqueous media. For use in other environments, SNP can be easily surface modified with appropriate groups.
Loading fluorescent dyes into the pores of silica nanoparticles or adsorbing them to the outer particle surface are far easier approaches than incorporating alcoxisilane-modified dyes into the silica structure. However, only the later strategy can effectively avoid the leaching of the dyes from the nanoparticles and the possible contamination of the samples with (often toxic) free dye molecules. The best strategy to prepare luminescent SNPs is thus to covalently link the dyes to the silica structure, not only because this avoids leaching of the dye molecules but also because it offers increased dye chemical and photochemical stability, as well as protection from enzymatic degradation in biological media. For example, the incorporation of different perylenediimide (PDI) derivatives by anchoring these dyes to the silica structure results in nanoreporters with excellent photostability and brightness. Laser scanning confocal fluorescence microscopy (LSCFM) images of the fluorescent silica nanoparticles show a uniform distribution of the dyes among the nanoparticles, with dimensions coinciding with those obtained by TEM, with tunable diameters from ca. 30 to 300 nm, and surface-decorated with tumor-targeting oligopeptides [38].

4. Emission Enhancement Using Plasmonic Nanoparticles

An alternative to producing vehicles with multiple dyes is to increase the amount of light emitted by each dye. This can be achieved by exploring the interaction between the dye exciton and the surface plasmon resonance (SPR) of noble metal nanoparticles. Gold nanoparticles (GNPs) are the most attractive for this application, as their SPR can be tuned in the visible/NIR region of the spectrum (by changing their shape), and they have good chemical stability and biocompatibility, while their surface can be easily modified. GNP-dye constructs can show enhanced brightness (also known as metal enhanced fluorescence, MEF) either due to an increase in the dye excitation efficiency, an increase in its emission quantum yield, or both [39][40][41]. The first phenomenon is related to the increased excitation rate of the dye due to the larger intensity of the local electromagnetic field at the metal surface. In the second case, electronic coupling of the dye and the metal nanoparticle leads to non-radiative transfer of excitation energy to the dye and transmission of the dye energy, as radiation, to the far field by the metal [42]. The two effects depend both on the dye-metal distance and on the spectral overlap of the metal SPR with the emission and excitation of the dye.
To obtain an enhancement of the dye emission, the distance between the dye and the metal nanoparticle should be as small as possible but larger than ca. 5 nm. This distance dependence results from the interplay of two opposite effects. On one hand, quenching of the dye emission by the metal occurs up to 5 nm from the metal surface [43][44] with the dye quantum yield being mostly suppressed for dye-metal distances below 5 nm, as shown by simulation results with a polarizable continuum coupled quantum mechanical model [45]. On the other hand, the electrical field intensity decays exponentially from the metal surface outward, with the enhancement decreasing as the dye-metal separation distance increases [46][47][48][49][50].
Dye-metal nanostructures with increased brightness thus require the use of spacer materials to control the metal-dye distance with precision. This has been attempted using silica [46][47][49][51][52], DNA [53], and polymers [47][54][55][56]. While DNA and polymer spacers can be too flexible to completely prevent dye-metal contact (leading to emission quenching), this problem can be minimized by using very high-density polymer brushes.
In another example, low-porosity silica was used as a rigid spacer to completely prevent contact between dyes and GNPs, offering excellent control over the dye-metal distance [46]. Other advantages of using silica to encapsulate the GNPs are its robustness, chemical stability, and versatility of surface modification (for example, for the conjugation of biomolecules or dyes [57]). To control the dye-metal distance, a hybrid system was developed with a GNP core and a silica shell of precise thickness. A perylenediimide (PDI) dye derivative with absorption and emission spectra overlapping the SPR of the GNPs and a very high fluorescence quantum yield was covalently attached to the silica outer surface. With this system, it was possible to precisely calculate the emission enhancement for different well-defined dye-metal constructs in dispersion [46].
Contradictory results on metal-enhanced emission abound in the literature, with metal-dye spacers as large as 90 nm described to produce emission enhancement (and other absurd effects). The reason for such inconsistency on the length scale associated with emission enhancement is probably related to experimental artifacts, such as light scattering and inner filter effects due to the metal nanoparticles. The real enhancement effect in the dye brightness can be obtained by correcting these artifacts [46][47].
Enhancing the brightness of individual reporters by coupling dyes to plasmonic nanoparticles can be extremely relevant to increasing sensitivity in sensing applications [58][59][60] or to being able to use dyes with inherent low brightness while offering other desirable characteristics. This is the case of gold nanoclusters (AuNC), with a diameter under 2 nm and size-dependent emission in the visible/NIR region. These are extremely photostable and biocompatible (depending on the stabilization ligands used), but they have poor colloidal stability and usually low brightness [61]. Co-encapsulation of AuNC and plasmonic nanoparticles in nanomaterials can enhance the AuNC brightness from both the AuNC–AuNP coupling and the large number of AuNC per reporter, as well as the colloidal and photo stability and targeting possibilities imparted by the nanomaterial carrier [62][63].

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Subjects: Nanoscience & Nanotechnology
Contributor MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
José Paulo Sequeira Farinha
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Revisions: 3 times (View History)
Update Date: 20 Dec 2023
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