Metal nanoparticles and their composites: a promising multifunctional nanomaterial for biomedical and related applications

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Metal nanoparticles and their composites: a promising multifunctional nanomaterial for biomedical and related applications

Vesna V. Vodnik and Una Bogdanovic´ ˇ Institute of Nuclear Department of Radiation Chemistry and Physics “GAMMA”, Vinca Sciences, University of Belgrade, Belgrade, Serbia

12.1 INTRODUCTION The dynamics of development technology in all science aspects moves forward very rapidly. Among the more transformative technologies, nanotechnology has the opportunity to offer the most, as it tends to create new materials with new properties and functionalities, such as nanomaterials. Their investigation is interesting from a technological standpoint since the transition from individual atoms to nanoscale material and the bulk state of matter have a great fundamental importance in science (Edelstein and Cammarata, 1996; Kreibig and Vollmer, 1995). A dramatic increase in the ratio of surface atoms to interior atoms with particle size decrease, accompanied by a great change of their physicochemical properties, is still the goal of today’s studies. Metal nanoparticles (NPs) are in the limelight of modern nanotechnology, especially colloidal Au, Ag, and Cu, which in transparent media provide a wide range of colors. They have been fascinating scientists for a long time due to very intense coloration, since their absorption and re-emission of light are dependent on the wavelength at which conduction electrons oscillate. For other metals (Pb, In, Co, Sn, and Cd) the plasma frequency lies in the UV region and NPs do not display strong color effects. Unlike them, noble metal NPs, as their size is reduced to tens of nanometers, exhibit a strong absorption band in the visible region, absent in the individual atom and in the bulk. The imaginary part of the dielectric constant at the plasmon frequency is very small and the near-field effect is so high that it makes the plasmon excitation of these NPs very interesting. Most commonly, surface plasmon resonance (SPR) frequencies are carried out with AuNPs, AgNPs, and CuNPs, and strongly depend

Materials for Biomedical Engineering: Inorganic Micro and Nanostructures. DOI: https://doi.org/10.1016/B978-0-08-102814-8.00014-7 © 2019 Elsevier Inc. All rights reserved.

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on their size, shape, interparticle distance, electron density, dielectric properties, and local environment (Mulvaney, 1996). Some of these interesting metal NPs properties, how they influence and interact with their surrounding, and how we can make these properties useful for different applications will be discussed in greater detail in this chapter. The first reason for studying metal NPs from a technological standpoint is their applications as optical systems. Their optical response appears when their surface plasmon excitations strongly couple with external light, which induces localization of SPR and consequently the electromagnetic field (EM) enhancement of plasmons. In particular, this optical signal enhancement allows the detection of Raman signals from a single molecule. The study of optical phenomena related to the EM response of metal NPs led to the development of the research field called plasmonics (Ozbay, 2006). From a plasma model, where the free electrons of a metal can be treated as an electron liquid of high density, it follows that electron density waves (called plasma oscillations or Langmuir waves) with an energy of the order of 10 eV will propagate along the interface of a metal. Interacting plasmons give rise to a wide variety of important properties of nanosystems, such as surface-enhanced Raman excitation (SERS), as one of the more important exploitations of metal NPs in biology, medicine, and engineering (Thomas et al., 2013; Hoppmann et al., 2013). Also, optical data communication and data storage need new materials with nonlinear optical phenomena. Tuning the optical properties with size and shape, together with the extremely high extinction coefficient of SPR compared with dye molecules have made them very interesting materials (Du et al., 1998). Plasmonics also allow new possibilities to treat cancer or the ability of using metal NPs as multimodal, intracellular optical sensors based on near infrared absorption or Raman scattering for biological and medical applications (Li et al., 2010; Gupta and Verma, 2014; Ravalli and Marrazza, 2015). Many sensitive and selective NPs have special physicochemical properties that offer a suitable platform for quantitation of metal ions, anions, proteins, and DNA, based on analyte-induced changes in their absorption, fluorescence, and scattering intensity, which is important in medical, environmental, material, pharmaceutical, and food science (Song et al., 2011). The successful applications of metal NPs in these fields require them to enter cells across the cell membrane with embedded or peripherally attached proteins (Rasch et al., 2010). These processes may be carried out through endocytosis, direct microinjection of NPs dispersions, by electroporation or a mediated uptake process using known biological interactions or promoters with surfacefunctionalized NPs (Medintz et al., 2005). As the NPs should be compatible with biological systems, their surface functionalization offers a convenient possibility for interaction with the cells. In addition, this process maintains good water solubility of NPs, their electrostatic/steric stabilization through the formation of a double electrical layer and binding organic molecules (biomolecules) to the particle surface. Also, the metal NPs could be embedded in polymer matrices in order

12.2 Some Interesting Properties of the Metals

to avoid their oxidation, aggregation/agglomeration, and the formation of the thermodynamically favored bulk material (Vodnik et al., 2013; Bogdanovi´c et al., 2015a,b; Stamenovic et al., 2018). The interparticle interaction in these nanocomposites enables them to act as molecular bridges in the polymer matrix. The correct choice of polymer, NPs type, and polymer NPs interaction, influence nanocomposite use in vastly differing applications. This chapter also discusses current efforts and research challenges in their emerging usage for potential biomedical and related applications. According to different applications of these nanostructures, their synthesis, size and shape control, surface modification, and characterization have attracted considerable attention from a fundamental and practical point of view. While it would be worth discussing the wide range of preparation methods more thoroughly, some of the synthetic procedures will be reviewed here, only to serve as a general introduction to the appropriate technique for preparation of nanomaterials with desired properties and property combinations. This chapter focuses on the characteristic properties of the metal NPs and their polymer-based nanocomposites which show pronounced features for biomedical and related applications, without going into biological or chemical detail. In addition, conjugation of metal NPs to biological molecules before their use for biological or medical applications will be also considered. The discussion will be illustrated with examples from the literature and from our experimental results. However, we have tried to summarize the most recent developments in the field of applied NPs, but due to the overwhelming nature and size of this research field, we only provided a cursory overview of some materials and thus apologize in advance for all omissions.

12.2 SOME INTERESTING PROPERTIES OF THE METALS ON THE NANOMETER LENGTH SCALE With characteristic physicochemical properties and promising applications in diverse fields, from the medical and pharmaceutical to the high-tech (Ruparelia et al., 2008; Grumezescu, 2016; Aiken and Finke, 1999), metal NPs are one of the most extensively studied colloidal systems in the nanoscience domain. Progress in this area depends on the possibility to synthesize stable metal NPs with different sizes and shapes since their electronic, optical, and catalytic properties are closely related to their morphology. With particles size decreasing to the nanometer scale, the motion of electrons is reduced, and surface effects become very important (Link and El-Sayed, 2003). The main feature of NPs significant for their properties and consequently various applications is their great surface-to-volume ratio. With decreasing particles size from macroscopic crystal to nanometer magnitude, the percentage of surface atoms increases—3.0 nm particles 35% of the surface atoms, while decreasing particle size to 0.7 nm increases this percentage to 92% (Aiken and Finke, 1999).

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Since large fractions of uncoordinated edge- and corner-atoms are located on the surface area, particles have high surface energy, they are highly reactive, and unstable, and should be stabilized/functionalized by different agents, which will be explained later. Additionally, the chemical reactivity of NPs is also affected by their morphology since their surface structure changes as a function of size and particles shape. Among the metal NPs, noble metals AuNPs, AgNPs, and CuNPs stand out, since they show specific behavior when exposed to light. Metal NPs have the ability to absorb/interact with electromagnetic radiation in a narrow wavelength range, which leads to collective oscillations of free conduction electrons in their surface. When these oscillations become resonant with the incident light, the absorption band called SPR appears in the absorption spectrum of metal colloid (Kreibig and Vollmer, 1995). These effects are more pronounced for noble metal NPs than for other metals, considering d d band transitions that push their plasmon frequencies into the visible part of the spectrum (Kreibig and Vollmer, 1995). The SPR band is directly influenced by particle size and shape (Fig. 12.1A) (Vodnik et al., 2008). With decreasing particle size, charge density increases, inducing changes to their electronic structure and optical properties. For small metal nanospheres, incident radiation induces a dipole mode that oscillates in phase with the electric field of the incoming light. This interaction is expressed as a narrow, intensive peak in the spectrum (Fig. 12.1A, Au nanospheres), that becomes wider and shifts toward higher wavelengths when the

FIGURE 12.1 (A) Size- and shape-dependent absorption spectra of AgNPs and AuNPs; (B) TEM images of metal NPs: AgNPs—nanospheres (a), truncated triangular nanoplates (b), dendrites (c), different shapes (d), nanoplates, discs, and rods (e); CuNPs— nanospheres (a), nanocubes (b); AuNPs—nanospheres with 17 nm (a) and 40 nm (b) diameters, and nanorods with different lengths (c,d).

12.2 Some Interesting Properties of the Metals

particles size is increased, due to light’s inability to homogeneously polarize larger NPs. In this case, higher-ordered modes are excited (Kreibig and Vollmer, 1995). With the introduction of the second, longitudinal dimension, particles shape becomes important for their optical properties. For nanorods, the resonance wavelength, and therefore absorption spectrum, depend on the electron oscillations across the transversal and/or longitudinal direction of rods (Momi´c et al., 2016). High-energy, transversal peaks originate from the electrons oscillating in the direction perpendicular to the major (long) rod axis, located at the same wavelength as the nanosphere of the same size, while the electron oscillations along the long axis are manifested as low-energy, longitudinal plasmon on greater wavelengths. In comparison to the transverse SPR wavelength, the longitudinal SPR peak is more sensitive to the changes in the dielectric properties of the surroundings and can be tuned by varying the aspect ratio of nanorods (Fig. 12.1A, Au nanorods). Further reduction of symmetry causes the appearance of new absorption bands. Due to dipole resonance in plane and quadrupole in and out of the plane, nanoprisms are indicated in the absorption spectrum by three peaks (Fig. 12.1A, AgNPs), and similarly, nanocubes by four (Vukoje et al., 2014a; Im et al., 2005). In concentrated colloid dispersions, when NPs are close to each other, their mutual interaction shifts SPR toward smaller energies together with the appearance of the additional band (Kreibig et al., 1981). The surrounding medium, and surfactant molecules/stabilizing agents on their surface have an influence on the nanoparticle optical properties as well (Vukovi´c and Nedeljkovi´c, 1993; Vukoje et al., 2012). Research within our group is partly related to the chemical synthesis of the metal NPs with various sizes and shapes. Their TEM micrographs are presented in the Fig. 12.1B. The electronic structure of metal NPs is also closely related to their catalytic properties. With great selectivity, efficiency, and recyclability, these particles have a great future as possible catalysts for various types of reactions, from ammonia detection to biosensing—detection of biomolecules (Gupta and Verma, 2014; Ravalli and Marrazza, 2015). The catalytic properties are influenced by their geometric structure, that is, size and shape, and crystallographic planes on their surface (Wang et al., 2011). Crystallographic planes have different surface energies that increase with charge density increase, when the fraction of edgeand corner-atoms is large. These uncoordinated atoms represent active places that could easily interact with surrounding molecules, enhancing the effectiveness of the catalytic reaction. Among metals, PtNPs, with certain characteristics that distinguish them from Au, Ag, and Cu, are the most commonly used as catalytic material (Nguyen and Minteer, 2015), but cheaper alternatives are noble metals (Okitsu and Mizukoshi, 2016). Knowing the overall properties of the metal NPs, their mutual interactions and interaction with surrounding molecules, whether they are capping or stabilizing agents, biomolecules, or electrolytes, allows one to design NPs with tailored properties that can be exploited for various purposes.

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12.3 NANOPARTICLE SYNTHESIS AND FUNCTIONALIZATION 12.3.1 SYNTHESIS APPROACHES TO METAL NANOPARTICLES The successful utilization of metal NPs in various applications depends on their size, shape, composition, and the surface functionalities. In order to extend the array of their properties, a large number of synthetic procedures is developed. Whether these methods are chemical or physical, the selection of the synthetic route depends on the desired characteristics. Physical synthetic procedures (evaporation/condensation, laser ablation) have some advantages over chemical methods in the form of easier control of experimental conditions, larger uniformity of metal NPs distribution, and the absence of solvent contamination (Oseguera-Galindo et al., 2016). On the other hand, since the equipment used in these methods is expensive and large, chemical methods are more affordable and accessible. They are based on chemical reduction of metal ions to metal NPs by an appropriate reducing agent, for example, sodium borohydride, sodium citrate dihydrate, hydrazine hydrate, etc. (Vodnik et al., 2008; Vukovi´c and Nedeljkovi´c, 1993; Turkewich et al., 1951; Bogdanovi´c et al., 2014b; Laban et al., 2016). Metal NPs characteristics, including the possibility and kinetics of particle formation, their size and shape, crystallinity, and stability, are affected by the characteristics of the reducing agent (primarily strength) and solvent (type, polarity). Lately, there has been a growing need for the development of eco-friendly or so-called “green” synthesis metal NPs, in order to reduce the use of toxic chemical agents. There is great potential in prokaryotic and eukaryotic microorganisms, marine organisms, extracts of biomolecules and plants, for use as reduction agents in metal NPs formation (Singhal et al., 2011; Jeevan et al., 2012). In addition, the literature offers many different types of metal NPs synthetic routes, such as radiolytic reduction of metal ions (Spasojevi´c et al., 2017), electrochemical and sonochemical reduction (Surudˇzi´c et al., 2013; Kumar et al., 2014), etc. A special class of nanomaterials with usually improved characteristics compared to metal NPs is represented by polymer-based nanocomposites. Besides the increased stability of metal NPs incorporated in polymer matrix, their electrical, optical and catalytic properties, antimicrobial activity, and sensing of biomolecules or metabolite products are more pronounced (Bogdanovi´c et al., 2015a,b; Stamenovic et al., 2018). On the other hand, metal NPs also have a positive effect on the polymer by increasing its thermal stability, mechanical, and electrical properties (Vodnik et al., 2011, 2013; Bogdanovi´c et al., 2014a, 2015b). Similarly to metal NPs, nanocomposites can be synthesized by physical (Feng et al., 2006) and chemical methods. The lack of physical synthetic routes (mechanical mixing of nanocomposite constituents) creates the great possibility of NPs aggregation and decomposition of the polymer matrix, whereby their main characteristics are lost. Much stronger binding between metal NPs and polymer matrix than hydrogen and van der Waals bonds characteristic for physically synthesized nanocomposites, is achieved by chemical and electrochemical reactions (Vodnik et al., 2011, 2013;

12.3 Nanoparticle Synthesis and Functionalization

Bogdanovi´c et al., 2014a, 2015b; Jovanovi´c et al., 2014). Depending on whether the metal NPs are already synthesized when the polymerization process is initiated (Bogdanovi´c et al., 2014a) or are formed together with the polymer (Bogdanovi´c et al., 2015a,b; Vukoje et al., 2014b), nanocomposite synthetic procedures can be divided into ex situ and in situ methods. In our research experiments, we have used both of these procedures for synthesis of metal NPs (Cu and Au) polyaniline (PANI) nanocomposites (Bogdanovi´c et al., 2014a, 2015a,b). The main advantage of in situ over ex situ methods is a decreased number of reactants that participate in the reaction—there is no need for additional capping/stabilizing agents for NPs protection nor for an oxidizing agent for the monomer polymerization, since metal ions have the role of oxidizing agent that will, for example, polymerize aniline to PANI, while simultaneously aniline monomers will reduce metal ions to NPs. Additionally, the formed polymer matrix stabilizes metal NPs and protects them from oxidation. One of today’s challenges in the nanotechnology field is designing new hybrid and advanced biomaterials that could be exploited for controlled drug delivery purposes, tissue engineering, bone and dental repairing/reconstructing, imaging, etc. Bionanocomposites may be able to fulfill the requirements needed for possible biomedical applications. In order to be used in medicine, bionanocomposites have to be biocompatible and nontoxic, biodegradable, and easily absorbed or eliminated from the body. The choice of an appropriate polymer matrix, nanofiller type, and their mutual interaction are important for the characteristics of the bionanocomposite as the final product with the desired characteristics, and for its further biomedical application. Natural polymers, such as polysaccharides, starch, and alginate (Boˇzani´c et al., 2011; Ghasemzadeh and Ghanaat, 2014), as well as synthetic ones that are water-soluble [poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), and guar gum] (Gupta and Verma, 2014; Vodnik et al., 2011, 2013; Ghasemzadeh and Ghanaat, 2014; Liu et al., 2012) or electrically conductive polymers (PANI, polyvinylpyrrolidone, and polypyrrole) (Bogdanovi´c et al., 2014a, 2015a,b; Stamenovic et al., 2014, 2018; Jiang et al., 2013) are used as polymer matrices in bionanocomposites, together with metal NPs as fillers. Nanocomposites based on metal NPs and the various aforementioned polymers can inherit characteristics of their constituent materials, including optical, electrical, or catalytic properties of metal NPs and thermal, mechanical, electrical, or chemical properties of polymers. However, due to the mutual interaction between metal NPs and polymer matrix, these characteristics can be modified, more or less pronounced, or express some new features.

12.3.2 FUNCTIONALIZATION OF METAL NANOPARTICLES: MANIPULATION OF NANOPARTICLES PROPERTIES In order to stabilize metal NPs and manipulate their properties at the same time, before their use for the desired applications, surface functionalization with organic

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molecules has been utilized as a powerful tool. This determines their physicochemical properties and could lead to processability, surface chemistry, and biocompatibility enhancement. The creation of specific sites on their surface is selective for molecular attachment and their interaction with the environment, and may yield a controlled assembly or the delivery of NPs to a target. Simple structures of NPs surrounded by functional molecules can be easily produced, and/or interparticle crosslinking can occur with other molecules. The core particle is often protected by a layer/several layers of molecules adsorbed or chemisorbed on their surface. The same layer might act as a biocompatible material, but more often an additional layer of linker molecules with reactive groups at both ends is required to allow further functionalization, depending on the application (Fig. 12.2). As the biological processes are typically performed in an aqueous environment, a hydrophilic NPs surface is desired for reactions with biological molecules. Functional molecules on NPs should prevent them from aggregation, maintain their good water solubility, retain their functionalities, and ensure biocompatibility before they interact with targeted subjects. Ligands containing amino or carboxy groups have been used to functionalize water-soluble NPs by electrostatic repulsion, and can be exploited for the conjugation of other molecules to the particles (Dojˇcilovi´c et al., 2016; Pajovi´c et al., 2015). In addition, metal NPs are not stable in micromolar concentrations in electrolytic solutions, which induce their aggregation. It can be avoided by adsorption of negatively charged phosphine molecules or nonthiolated ssDNA on the AuNPs surfaces (Liu, 2012; Koo et al., 2015). The functionalization of NPs can be accomplished during their synthesis by a suitable agent. Among the surface coatings there is an increasing interest in using polysaccharides as linear or branched polymeric carbohydrate structures, for tailoring NPs surface functionality (Habibi and Dufresne, 2008). In this case, carbohydrates as biomimetic functional molecules on the NPs surface can be used for the diagnosis and treatment, as a carrier for anti-HIV prodrugs (Chiodo et al., 2014), or for detecting carbohydrate-binding proteins (Adak et al., 2014). Various organic molecules with different features can be exploited for NPs functionalization or spatial assembly, bringing the unique properties and functionality of both materials, NPs and biomolecules. Proteins, enzymes, DNA, RNA, lipids, vitamins, peptides, and water-soluble polymers are able to stabilize and functionalize NPs without undesirable consequences on the environment and biosystems. This functionalization could be performed through chemisorption of thiol groups, by electrostatic adsorption of positively charged biomolecules on the negatively charged NPs surfaces or vice versa, and covalent/noncovalent binding between biomolecule functional groups and particles. DNA and RNA can be employed as generic polymeric molecules to functionalize AuNPs in an aqueous solution by a thiol metal bond, induce AuNPs assembly into aggregates, or organize them into spatially defined

12.3 Nanoparticle Synthesis and Functionalization

FIGURE 12.2 Schematic representation of metal NPs conjugation with functional molecules and polymers and their applications.

structures (dimers, trimers) (Herne and Tarlov, 1997; Mirkin et al., 1996; Alivisatos et al., 1996). Selective sensitivity of the AgNPs can be improved by their functionalization with protein cytochrome c, while the specific uptake of AuNPs by cells is optimized by their conjugation with the corresponding

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peptide (Sivanesan et al., 2011; Nativo et al., 2008). Efficient stabilization/ functionalization of AuNPs could be performed by enzymes, α-amylase, or lysozyme (Rangnekar et al., 2007; Eby et al., 2009). Biomacromolecules and fluorescent dyes have also been used to functionalize nonfluorescent metal NPs to form fluorescence for detection proteins, DNAs, cells, and bacteria or to determine the activity/concentration of enzymes (Laban et al., 2016; Dojˇcilovi´c et al., 2016; Pajovi´c et al., 2015; Tseng et al., 2014; Laban et al., 2014). Metal NPs functionalization includes ampicillin and tetracycline molecules (Brown et al., 2012; Buszewski et al., 2016), when these systems become potent bactericidal agents with unique properties that subvert antibiotic resistance mechanisms of multiple-drug-resistant bacteria. Besides biological molecules, various polymer molecules can also be used to prevent their aggregation and functionalize them for a great variety of purposes. This combination results in the development of new functions that are noncharacteristic for individual components and represents a simple way to use their advantages. For example, NPs could be stabilized by surface-active polymers adsorbed strongly onto their surface due to van der Waals attractive forces, since there is a large decrease in their surface energy in comparison with native NPs (Rozenberg and Tenne, 2008). The interaction between NPs in these nanocomposites enables them to act as molecular bridges in the polymer matrix. Of the polyesters that show promise in biomedical fields, PEG is the most prevalent. Due to its amphiphilic properties, PEG-functionalized metal NPs can be soluble in a number of solvents, with intermediate polarity, and can be used for biological, chemical, and biomedical applications (Liu et al., 2012). Another hydrophilic and nontoxic polymer discussed in the literature is chitosan, which is able to stabilize metal NPs and provide them with antibacterial activity (Vukoje et al., 2014a; Gu et al., 2014).

12.4 APPLICATIONS OF METAL NANOPARTICLES AND THEIR POLYMER-BASED NANOCOMPOSITES In the previous sections we noted that successful utilization of metal NPs in biology and biomedicine, and interdisciplinary fields, critically depends on their structural features and surface chemistry. Desirable NPs characteristics can be achieved before their use, and different environmentally friendly methods have been developed. Particularly, great progress in the manipulation of NPs in the last decades has allowed us to encroach into the fascinating world at the length scale of molecules and DNA strands. Moreover, the possibility to concentrate, amplify, and manipulate light at the nanoscale level gave rise to the idea to use metal NPs in cells. With the biological size, comparable to proteins (B5 nm) and DNA chains, but smaller than cells, bacteria, and viruses, metal NPs have found applications in various fields (Fig. 12.2).

12.4 Applications of Metal Nanoparticles

In the following sections, we give selected examples of different types of these nanosystem applications that have been reported recently, discussing their challenges and perspectives.

12.4.1 MEDICAL APPLICATIONS As mentioned above, NPs have a biological size and their successful applications in medicine sometimes require them to enter cells across the cell membrane. Surface functionalization of NPs offers mediated/targeted uptake by the cells using known biological interactions. These reactions can take place over biomolecules such as antibodies, collagen, glutathione, fluorophores, etc., depending on the function required by the application.

12.4.1.1 Cancer immunotherapy/drug delivery In the last few years, significant progress has been made in the field of cancer immunotherapy. The goal is targeting immune-suppressive populations and stimulating immune effector cells against cancer. Recent investigations have shown that the delivery and efficacy of immunotherapeutic agents and molecular therapies can be enhanced, together with reduced adverse outcomes, through the use of NPs. They have been explored as immunotherapy carriers due to their preferential accumulation within tissues and cells of the immune system. AuNPs have been applied as a promising carrier for immune therapies, including cancer antigen and immune adjuvant delivery (Lin et al., 2013a,b). Their size- and shape-dependent optical properties can be exploited in photothermal ablation and light-triggered drug delivery (Arvizo et al., 2010). Exactly these characteristics affect blood clearance and organ accumulation of AuNPs in vivo, that is, the smaller NPs circulate in the blood longer and can be distributed more widely than larger NPs (Lin et al., 2013a,b; Arvizo et al., 2010). Their simple surface coating with PEG can reduce opsonization and uptake by the reticuloendothelial system. However, as the PEG molecules can be displaced with cystine present in the blood (causing protein absorption and macrophage uptake), this problem was solved by adding an alkyl linker between the PEG and the thiol group bound to the AuNPs surface (Larson et al., 2012). On the other hand, AuNPs blood half-life increases with decreasing NPs size and increasing PEG molecular weight (Perrault et al., 2009), while a higher percentage of smaller PEGylated AuNPs reaches the targeted tumor site, but accumulate in the liver and spleen to a lesser extent than larger ones (Zhang et al., 2009). In addition, positively charged NPs were taken up to a much higher extent by nonphagocytic cells than negatively charged ones (Liu et al., 2013). Varying the hydrophobicity of 2 nm AuNPs, Moyano et al. determined that more hydrophobic NPs would induce higher expression of inflammatory cytokines by mouse splenocytes (Moyano et al., 2012). Furthermore, NPs can be used for photothermal therapy (PTT) by varying their size, shape, and shell thickness, to absorb light in the near-infrared (NIR) range. Among the large variety of NPs shapes, nanorods, nanocubes, and

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nanocages would have large absorption cross-sections and display a large photothermal effect with absorbed photons, which can be converted into phonons— lattice vibrations to produce a localized temperature jump. Within the NIR range (where blood and tissue are relatively transparent), the NPs show strong SPR absorption of light which has penetrated through the healthy tissue, then they heat up and destroy the nearby cancer cells (Kennedy et al., 2011). Based on this effect, conjugate Au nanocages and tumor-targeting antibody (anti-HER2) were used to destroy breast cancer cells in vitro owing to the large absorption crosssection of particles that facilitates the conversion of NIR irradiation into heat (Chen et al., 2007). Bear et al. exploited Au nanoshells in the elimination of metastatic melanoma by PTT, which can promote a tumor-specific immune response against a distant, subcutaneous B16-ovalbumin (B16-OVA) tumor (Bear et al., 2013). They found that a combination of PTT with adoptive T-cell therapy could inhibit metastatic tumor growth to a greater extent than either treatment alone. Another potential combination PTT with immune modulatory agents such as drugs, oligonucleotides, or proteins together with optically and thermally responsive metal NPs in delivering such agents, could be exploited. For instance, macrolide-coated, NIR tuned Au nanorods target tumors in vivo as a combination PTT and immunotherapy (Dreaden et al., 2012), while doxorubicin-loaded hollow Au nanoshells apply a combination PTT and chemotherapy treatment in vivo (You et al., 2012). This system could potentially be applied in a metastatic disease model, when drug and the thermal treatment could induce a systemic immune response against distant, untreated sites. A number of combination treatment possibilities of thermally responsive metal NPs, such as a combination of optical contrast enhancement and the photothermal effect, or thermal ablation or light-triggered release which can be combined with delivery of nucleic acid immune adjuvants, have been previously reviewed (Almeida et al., 2014; Zeng et al., 2014). Thus, metal NPs, especially AuNPs, may act as a new class of anticancer nanomedicine agent, combining therapy and diagnosis. Moreover, as mentioned above, metal NPs are attractive candidates for adjuvant delivery and safety of immunotherapy agents. For example, several groups have investigated AuNPs-mediated CpG oligonucleotide delivery as short DNA sequences that mimic bacterial DNA and thus stimulate immune cells via interaction with toll-like receptor (TLR9) (Lin et al., 2013b) or they have investigated AuNPs vaccines for their ability to deliver a large payload which recently have been used in an HIV model or as a cancer vaccine platform (Safari et al., 2012).

12.4.1.2 Imaging of tissues and cells/nanoparticles in diagnostics Metal NPs have been in active use in cell imaging owing to their size-dependent efficiency of absorption or scattering light. The larger NPs exhibit more scattering, while smaller ones exhibit higher absorption cross-sections. Thus, their size and absorption/scattering cross-sections should be carefully considered when designing a cell imaging system. The larger metal NPs ($30 nm) are ideal for cellular labeling using dark-field microscopy, while small NPs (,10 nm) can be

12.4 Applications of Metal Nanoparticles

used in photothermal techniques. Cellular labeling using NPs offers more possibilities for quantitative imaging of biological molecules (i.e., DNA, proteins, viruses, etc.) in the cell’s endogenous environment or detection of biospecific interactions. They can be used as in vitro and in vivo imaging contrast probes. Their bioimaging relies on intense Rayleigh/Mie light scattering from their surface, providing very bright and stable light-scattering signals which are important for long-term imaging of living cells, the plasmonic field near their surface or between coupled NPs, which enhance the Raman signals of molecules near this fields, allowing monitoring of the changes in the molecular environment around the NPs, the enhancement or quenching of the fluorescence of the molecules in the vicinity of the NPs, and on the SPR mechanism that produces multiple effects, that is, photothermal, photoacoustic, etc., for multimodal single-cell imaging. Numerous reports have appeared describing the use of Au particles of different sizes and shapes for real-time detection of their penetration into living cells, which permits extensive visualization of the cells under continuous illumination to achieve multicolor imaging (Austin et al., 2014). The light-scattering effect of metal NPs can be used to differentiate diseased cells (i.e., cancerous) from the healthy cell population, for molecular recognition applications, and in the identification of cell-surface receptors (Austin et al., 2015). El-Sayed’s group employed plasmonically enhanced Raman and Rayleigh scattering from AuNPs to identify human oral cancer cells and to selectively destroy them with AuNPs-assisted photothermal therapy (El-Sayed et al., 2006). Other groups have used various antibody-labeled NPs in conjunction with SERS, spectral and fluorescence imaging to selective label and visualize cancerous cells, both in vitro and in vivo (Seekell et al., 2011; Lee et al., 2014). Moreover, Kneipp et al. used SERS and surface-enhanced hyper-Raman scattering demonstrated to probe and image the pH in live cells by nonspecific intracellular AuNPs (Kneipp et al., 2007). These NPs can target mitochondria (Wang et al., 2011; Ju et al., 2014a) or they have been used to probe intracellular events involved in cell growth and death (Kang et al., 2012). This platform further allows to determine the drug efficacy of chemotherapeutics and monitor apoptotic molecular events in real-time, when AuNPs have been coupled with fluorescence to monitor the induction of apoptosis and caspase biomarkers due to energy transfer between NPs and target acceptor molecule (Chen et al., 2014; Wen et al., 2014). This AuNPs fluorescence imaging platform was also used for the real-time tracking of oxidative stress levels in both in vitro and in vivo models (Ju et al., 2014b), to detect two forms of tumor mRNA in breast cancer cells (Qiao et al., 2011) or to monitor enzymatic activity and cellular metabolism (Wu et al., 2013; Han et al., 2014). Future efforts in using plasmonic NPs, especially AuNPs, toward real-time single-molecule mapping within single cells should involve higher imaging resolution, by using smaller NPs and detecting the light absorption instead of scattering using photothermal imaging, together with novel bioconjugation strategies to target specific molecules in single cells and to provide complex information from the cell sample (El-Sayed et al., 2006; Leduc et al., 2013).

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12.4.2 APPLICATIONS IN BIOLOGY Due to their properties, especially small size and great surface area, metal NPs and their composites with various natural and synthetic polymers have great potential for biological applications, either for detection and imaging of biomolecules, or as an antimicrobial agent. Functionalization of metal NPs or their incorporation in polymer matrix enable their easy interaction and bonding with different functional groups present in the biomolecules or on the surfaces of cells, which is the basis for their biological application—labeling and detection of the specific molecules.

12.4.2.1 Fluorescent biological labeling Imaging of living systems, specific cells/tissues, or biomolecules, based on fluorescence, is very sensitive and selective toward certain chemical and/or biological molecules that fluoresce initially or are labeled by fluorescent species, NPs, or sensors, and is not harmful toward living organisms. Incorporation of fluorescent NPs into cells or tissues enables their easy visualization, while NPs functionalized by proper oligomers/polymers, ligands, or antibodies can be used for targeted imaging of biomolecules. The possibility of noble metal NPs functionalization by fluorescent molecules, together with their characteristic features, make them applicable to cellular imaging. Unlike fluorescent probes (organic fluorophores), metal NPs have good photostability, strong photoluminescence, high emission rate, and sizedependent tunable fluorescence, that make them very attractive and suitable for biological labeling. In addition, varying surface species is a simple way to optimize the intensity of fluorescence and to design metal NPs with specific spectral properties. Fluorescent metal NPs could be used to illustrate the dynamics of the intercellular network. Fluorescent AuNPs showed great potential as markers for in vitro and in vivo cell targeting (Shang and Nienhaus, 2012) and they could be used as a biocompatible fluorescent probe. Furthermore, in order to explore and clarify metal NPs uptake by human cells, prior to their biomedical use, live HeLa cells were exposed to lipoic acid-protected AuNPs (Yang et al., 2013). Knowing the uptake kinetics, intercellular localization, and the uptake path is crucial for metal NPs utilization for drug delivery and cell diagnosis in a controllable and biocompatible manner. For example, Duan et al. synthesized biocompatible Au nanoclusters coated with chitosan-N-acetyl-L cysteine which showed low cytotoxicity, allowing their usage for living cell imaging (Duan et al., 2018). Also, AuNPs showed great potential for diagnostic purposes, as a fluorescence probe for the selective determination of glutathione, in living cells and human blood (Tian et al., 2012), or to label hematopoietic cells (Huang et al., 2011). The folic-acid-conjugated fluorescent AuNPs could be used for folate receptortargeted imaging of oral squamous cell carcinoma and breast adenocarcinoma cell MCF-7 (Retnakumari et al., 2010). Silica-coated AuNPs with incorporated

12.4 Applications of Metal Nanoparticles

thrombin-activatable fluorescent peptide showed great potential for in situ detection of a thrombotic lesion in a mouse model, as well as for possible therapy in clinical applications (Kwon et al., 2018), while a nanohybrid of Au nanorods and carbon dots with silica as a bridge between them were successfully used for in vitro detection of synthetic macrophages, enabling their possible application as a theranostic contrast agent for atherosclerosis (Liu et al., 2018). Our previous investigations in this field showed that AuNPs and AgNPs, functionalized by tryptophan and riboflavin, could be used for deep UV fluorescence labeling study of microbial cells (Dojˇcilovi´c et al., 2016; Pajovi´c et al., 2015). Nanocomposites of metal nanoparticles with polymers also could find their role in cell imaging. PVA-borax hydrogel functionalized by Ag dots was used as the sensing probe to detect insulin in human blood, while the three-layer core shell nanostructure of AgNPs, silica, and fluorescent dye, is highly sensitive to prostatespecific antigen in the human serum (Pourreza and Ghomi, 2017; Xu et al., 2017). Besides their biocompatibility, metal NPs need to target specific tissues with high selectivity in vivo, and to be efficiently released from the living organism by the metabolic system, while their long-term influence on the cells and their cytotoxicity, should be clarified.

12.4.2.2 Biodetection of proteins Interesting optical properties of metal NPs enable their possible application as sensors of various molecules, including biomolecules. Great chemical reactivity of metal NPs, based on functional groups present on their surface, makes them very useful for detection of different proteins, nucleic acids, and metabolites. These sensors work on the key lock principle, whereby properly functionalized metal NPs can selectively conjugate with the desired analyte (biomolecule). For example, AuNPs could be used for malaria antigen detection, sensing of aflatoxin, and determination of the content of hCG hormone from human blood (Guirgis et al., 2012; Wang et al., 2014; Schneider et al., 2000). The designation of DNA sequences is of great concern in modern diagnostic methods. Among noble metal NPs, AuNPs have potential to be used as a biochip for precise DNA sequence determination. Because of their high stability, resistance to agglomeration, and easy preparation, AuNPs functionalized with oligonucleotides are usually used for DNA detection (Ma et al., 2018). Several methods have employed AuNPs for DNA detection—colorimetric, electrical, SERS, etc. Colorimetric DNA detection is based on NPs’ color change, that is, SPR shifting. Targeted DNA is complementary to the DNA chains from the particle surface, which in turn results in their interaction (bonding), and finally, in the formation of NPs aggregates (Nourisaeid et al., 2016). The lower sensitivity of the colorimetric DNA detection method is overcome by the electrical method, whereby the interaction between targeted DNA and corresponding metal NPs is manifested by the change of the electrical signal (Song et al., 2011). Furthermore, based on the strong electric field generated on the metal NPs surface, they have great potential for the SERS technique which provides molecular fingerprints. There is a signal

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enhancing when NPs aggregate upon bonding to targeted DNA or they are functionalized with oligonucleotides that contain Raman-active dyes for identification (Lin et al., 2015; Xu et al., 2015). Also, nanocomposites based on metal NPs and polymers can be used for DNA detection (Zhu et al., 2015; Jin et al., 2018). AuNPs and AgNPs, of different sizes, were used for the detection of different proteins (Li et al., 2017; Zong et al., 2017). As bare and unmodified NPs, they easily can interact with proteins, at the same time as receptors and indicators (Saptarshi et al., 2013), which make them a simple sensing material for colorimetric detection of proteins. With the change of NPs species and their size, as well as the protein and their concentration, particular absorbance responses were created, together with a visual color change. This enables simple protein differentiation and potential applications in medical diagnosis (Sriram et al., 2015). Biodetection based on noble metal NPs as a novel diagnostic method for detection of various biological molecules can provide vital information in considering many diseases. Besides demands for higher selectivity and sensitivity toward key biomolecules, with minimal cost, there is a need for real-time and simple detection.

12.4.2.3 Biosensing applications Metal NPs represent receptors for various biological species—bacteria and viruses, DNA, or proteins from antibodies and antigens, to more simple molecules, like glucose, acting as a connection between nanotechnology and biotechnology. To be considered as a reliable biosensor of a certain analyte, it needs to effectively display (optical/electrical signal) its interaction with required molecules, even when they are in traces. Using NPs as biosensors enables increasing the sensitivity toward biomolecules, as well as lowering the detection limit, even to an individual molecule. Among noble metal NPs, AuNPs have been commonly used as biosensors. Since their optical properties are environment-dependent, the changes to particle surfaces through interaction with the surrounding molecules induces a change in colloid color that can be observed by the naked eye, together with changes in the SPR position, shape, intensity, etc. This feature is the basis for immunosensors (Lesniewski et al., 2014; Liu et al., 2015) or for efficient colorimetric biosensing of nucleic acids (Zaher et al., 2018). For example, AuNPs modified with covalently bonded anti-T7 antibodies serve as a simple and selective colorimetric immunosensor for T7 bacteriophages, as a model organism for adenoviruses (Lesniewski et al., 2014). T7 bacteriophages form a complex with modified AuNPs, causing their agglomeration, and consequently a change in their color and SPR. Also, this investigation pointed out that AuNPs as immunosensors can detect all T7 bacteriophages present in the sample, compared to biological tests that label only biologically active ones. Similarly, modified AuNPs could be used as colorimetric immunosensors for influenza A virus (Draz and Shafiee, 2018). Besides AuNPs, AgNPs and CuNPs also have the possibility to be used as colorimetric immunosensors for respiratory syncytial virus (Valdez et al., 2016). This

12.4 Applications of Metal Nanoparticles

type of virus detection has an advantage over classic immunoassays, since it is a simple and one-step sensing technique, and does not require additional amplification. Similarly, pathogen microorganisms, such as Escherichia coli, of which the enterohemorrhagic serotype is responsible for the production of shiga-like toxin that causes bloody diarrhea in humans, or Lactobacillus species and botulinum neurotoxin, could be detected by a colorimetric biosensor based on AuNPs (Jyoti et al., 2010; Verdoodt et al., 2017; Liu et al., 2014). AuNPs immobilized by appropriate bioreceptor units are used for the detection of electroactive biological species, viruses, hormones, and cancer biomarkers, whereby modified AuNPs transfer electrons between the biomolecule and electrode, catalyzing their oxidation or reduction (Ravalli and Marrazza, 2015; Chandra et al., 2013; Alipour et al., 2013; Mazloum-Ardakani et al., 2015; Arya et al., 2011). As a response to this process, the electrical signal appears as amperometric, potentiometric, or impedimetric. Besides AuNPs, several studies have pointed out the significance of other metal NPs for the detection of various biomolecules (Xu et al., 2006; Wang et al., 2016).

12.4.2.4 Antimicrobial testing With microbes evolving, and their rising resistance to commonly used antimicrobial agents (antibiotics and antimycotics), there is a need for the development of new antimicrobial materials. Hence, science today is also aiming to find new, useful, and efficient antimicrobial agents (Bogdanovi´c et al., 2014b; Theron et al., 2008; Giannossa et al., 2013). With the emergence of nanotechnology and the possibilities that it offers, metal NPs have found their place as antimicrobial agents in various fields, from medicine to food industry and wastewater treatment (Theron et al., 2008; Giannossa et al., 2013). Their great characteristics in this area were recognized even in ancient times, although they were not known as NPs (Lemire et al., 2013). Ag and Cu have proved to be excellent antimicrobial materials, and could be used for this purpose in the form of NPs, either alone or incorporated in the polymer matrix, in the form of ions or hybrid structures. The exact mechanism of their antimicrobial activity is not completely known yet. It is assumed that, when in contact with microbial cells, AgNPs are incorporated in the cell membrane, or they penetrate, causing damages and leaking of cellular content, and finally cell death (Cho et al., 2005). Similarly, when CuNPs are in a medium with microbes, there is a release of copper ions (Cu21 and Cu1) from the particle surface, together with cyclic redox reactions between them on the microbial cell surface, which cause its destruction, and finally penetration of copper into the cell. Microbes’ cell function is disrupted in multiple ways, such as permeability, respiratory and biochemical functions together with DNA damages (Ruparelia et al., 2008). The most important property of these NPs is their ability to participate in redox reactions, representing catalytic cofactors for microbial cell enzymes that can generate or catalyze reactive oxygen species, responsible for cell oxidation and damage (Lemire et al., 2013). Also, a term called “ionic/molecular mimicry”

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is described as the displacement of original elements (sulfur, nitrogen, oxygen) from microbe biomolecules by Cu ions, for example, that form organic complexes with these elements, causing changes to osmotic balance and deformations to proteins and nucleic acid structure (Lemire et al., 2013). Metal NPs are suitable as an antimicrobial material, since their small size fits the size of biomolecules, which makes their mutual interaction possible and easy. In addition to their size, shape and surface structure, characteristics of microbes— type, structure, as well as the conditions of their exposure to the antimicrobial agent (time, temperature, pH, etc.)—are also important for examination of material antimicrobial activity (Raza et al., 2016). Based on literature data, AuNPs and PtNPs did not show any antibacterial activity as bare NPs, but in the form of bimetal NPs, functionalized or in composites, their antimicrobial efficiency becomes evident (Boomi et al., 2014; Zhang et al., 2015). Our researches are related to the examination of antimicrobial activity of AgNPs and CuNPs, and their composites (Bogdanovi´c et al., 2014b, 2015a; Bogdanovi´c et al., 2018; Ili´c et al., 2010; Lazi´c et al., 2013). An interesting and very intensive antimicrobial property was observed for CuNPs (B6 nm) whether bare or incorporated in conducting polymer PANI (Bogdanovi´c et al., 2014b, 2015a; Bogdanovi´c et al., 2018). Some of these results are presented in Fig. 12.3,

100 90 80 Cell reduction (%)

414

70 60 50 40 30 20 10 0

E. coli

S. aureus 8 ppm

16 ppm

C. albicans 32 ppm

FIGURE 12.3 CuNPs concentration-dependent reduction ability on E. coli, S. aureus, and C. albicans over 2 h incubation time.

12.4 Applications of Metal Nanoparticles

as quantitative (chart) measurements for bare CuNP antimicrobial activity, and qualitative, AFM measurements of the microbial morphology deformations after their exposure to Cu/PANI nanocomposites (Figs. 12.4 12.6). This system could find practical applications in the rapid control of microbial infections in contaminated water before further processing. In addition to the previous study, many other nanocomposites based on AgNPs and polymers with excellent antibacterial behavior against some representative bacteria, fungi, and foodborne pathogens have also been reviewed (Link and El-Sayed, 2003; Zare and Shabani, 2016; Solairaj and Rameshthangam, 2017). Another attractive research area in progressive applications of metal NPs is in the production of antimicrobial textiles (medical textiles, sportswear, and everyday clothing). A major work on the functionalization of these materials has been done with AgNPs due to their extraordinary efficiency against microbes applied to the surface or incorporated into textile fibers, providing a product which kills/ inhibits microbial growth. Our recent studies reported the intrinsic antimicrobial efficiency of AgNPs deposited on different textile materials (Ili´c et al., 2009, 2010; Lazi´c et al., 2012). Overall, the multiple functionalities of metal NPs make them attractive for various approaches in this field, such as introducing NPs onto textile and different surfaces, or using them in water treatment. However, in order to be safely used, metal NPs need to be minutely characterized. The mechanisms of their

FIGURE 12.4 AFM images of E. coli (A) before and after incubation with Cu/PANI (20 ppm) for (B) 1 h and (C and D) 2 h.

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FIGURE 12.5 AFM images of S. aureus (A) before and after incubation with Cu/PANI (20 ppm) for (B) 1 h and (C and D) 2 h. ˇ Reprinted with permission from Bogdanovic´ U., Vodnik V., Mitric M., Dimitrijevic S., Skapin S.D., ˇZunicˇ V., et al., Nanomaterial with high antimicrobial efficacy-copper/polyaniline nanocomposite, ACS Appl. Mater. Interfaces 7, 2015a, 1955 1966. Copyright (2015) American Chemical Society.

toxicity and safe exploitation of their antimicrobial properties without negatively impacting human health and the environment need to be well examined and understood.

12.5 CONCLUSIONS AND OUTLOOK Exceptional accomplishments are performed in designing nanomaterials with different functionalities that can be used for various medical and biological applications. Considering unique size- and shape-dependent physicochemical properties, metal NPs are at the forefront of scaffolds for designing such bioactive materials of the novel opportunities as diagnostic, delivery, and disease-treated systems. Thus, in vitro applications of metal NPs are well established, while in vivo ones

12.5 Conclusions and Outlook

FIGURE 12.6 AFM images of C. albicans (A) before and after incubation with Cu/PANI (20 ppm) for (B) 1 h and (C and D) 2 h. ˇ Reprinted with permission from Bogdanovic´ U., Vodnik V., Mitric M., Dimitrijevic S., Skapin S.D., ˇZunicˇ V., et al., Nanomaterial with high antimicrobial efficacy-copper/polyaniline nanocomposite, ACS Appl. Mater. Interfaces 7, 2015a, 1955 1966. Copyright (2015) American Chemical Society.

show promising results. However, their application requires a directed design, providing actuation and stability in complex environments, such as living organisms. Among these processes, their surface functionalization by known biological interactions or promoters holds great promise and offers a compatible surface (in addition to the required good water solubility) to interact with cells before realizing their own functionalities. By appropriate modification, specific binding to target surfaces, cells or organelles can be tuned, for the controlled targeting of metal NPs. Up to now, highly attractive features such as biosensing capabilities in connection with SERS imaging, novel therapies based on local drug delivery, and photothermal therapy, are being explored. Their future applications should be considered for actual biological problems and the molecular precision of the interaction between them and intracellular subjects, which should lead to novel routes in biomedical practice.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia (Grants 172056 and 45020).

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