Introduction

Natural fibers and textiles have been used by humans since ancient times. Our ancestors used fur and animal skin for dressing and protection from the environment in those times, but very soon they started to use vegetal fibers to make fabrics. Nowadays, textiles have not been the things to only cover the human body, but also added choices of fast-growing population and developments needed technologies to be introduced and developed. There is a new revolution on the textile industry with the evaluation of new technologies that added special functions and properties to the fabrics such as antimicrobial, UV protection, antioxidant, flame retardant, coloration, etc. [1]. Textile industry centrally interferes in the economy of the region because of its contribution to the industrial output, employment generation, and foreign exchange earnings.

A rapid growth in textile materials production for medical, healthcare, hygiene, and sports applications renewed the interest for antimicrobial functionalization in previous years globally [2]. There are harmful effects of the microbial growth on textile materials as well as the adverse effects on the wear comfort. Various antimicrobial agents from metal salts to inorganic salts and organic compounds have been used in antimicrobial treatment of textile materials [3].

Nanotechnology is increasingly attracting worldwide attention not only of scientists and researchers but also industries, due to their huge economical potential owing to the unique and new properties of nanomaterials. Nanotechnology has real commercial potential for the textile industry also due to the dominating effects of nanomaterials on textiles in comparison to other conventional methods in terms of permanent effects and retention of the functionalities after laundering or wearing [4]. Materials of the size in range of 1–100 nm (10−9 m) are studied as nanomaterials and designing of them as nanotechnology. Nanotechnology and nanomaterials find their applications in every aspect of life such as medicines, electronics, smart devices, and textiles. Nano-sized materials have a higher efficiency than bulk materials owing to larger surface area per unit mass. Size and shape are the primary characteristics of nanomaterials responsible for the applications imparted by them. The way of synthesis or fabrication methods and the reducing or stabilizing agents determine the shape and size of nanomaterials lead to specific characteristics of synthesized nanomaterials. Nanotechnology has made it possible to produce a new generation of antimicrobial textiles by innovative finishes of the fabric surface in the textile industry. Nanoparticles are used in textile finishing to alter surface properties and to provide textile functionalities. Smart textiles, thus developed via nanotechnology applications, can widely be used for wound healing and medical applications in hospitals [5].

Nanomaterial for Textiles

Inorganic and metallic-based nano-structured materials have created a new interesting field in all branches of science for continuous investigations, because of their undeniably unique properties. On the basis of their elemental compositions, nanomaterials can also be categorized into metal nanoparticles; metal oxides and nano-composites for textile functionalization (Fig. 1). Synthesis by chemical means, in situ synthesis on textiles, and various wet processes are the different means toward textile applications [6].

Fig. 1
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Categories of nanomaterials on textiles

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Transition Metals (Ag, Au, etc.)

Silver and gold nanoparticles are highly studied and popular among transition metal nanomaterials. Silver has been used in the past for the treatment of burns, wounds, and several bacterial infections in the form of metallic silver, silver nitrate, and silver sulfadiazine. Metallic silver in the form of nanoparticles has made a remarkable position as a potential antimicrobial agent. Since some of the antibiotics have been avoided due to developed resistance of several pathogenic bacteria, antimicrobial silver nanoparticles find their place highly important. These remarkable properties of silver nanoparticles make their use in dressings, medicinal devices, etc. [7]. Along with the antimicrobial activity, silver nanoparticles exhibit a variety of brilliant hues, which can ultimately be imparted on textiles, owing to the surface plasmon resonance, and the color observed is dependent on the size and shape of the nanoparticles [8]. A lot of studies have been carried out for textile functionalization via silver nanoparticles. Silver nanoparticles were synthesized and deposited on different types of fabrics (nylon, polyester, and cotton) using ultrasound irradiation (sonochemical coating). Excellent antibacterial activity results of the Ag-fabric composite against Escherichia coli (gram-negative) and Staphylococcus aureus (gram-positive) cultures were observed [5]. Ag nanoparticles with high density coating were synthesized through potassium hydroxide pre-activation and in situ reduction of silver nitrate on the surface of cotton fabric. Ag nanoparticles-treated fabric showed hydrophobicity with contact angle of more than 150°. High antibacterial activity and excellent ultraviolet protection of the fabric with good durability against repeated launderings were demonstrated simultaneously [9].

Gold nanoparticles also had been studied for their colorimetric and antimicrobial properties on textiles. Gold nanoparticles were developed via a simple in situ synthesis route (silk fabric immersed in heated Au precursor solution) for multifunctional silk fabrics. Localized surface plasmon resonance property of gold nanoparticles colored the silk fabrics into red and brown colors. The silk fabrics treated with gold nanoparticles showed good light fastness along with strong antibacterial activity, excellent UV protection property, and enhanced thermal conductivity [10]. In another study, gold nanoparticles were synthesized on bamboo fibers in alkaline medium, and the reduction was carried by bamboo fibers themselves under heating conditions. Light and rub fast colors were obtained on bamboo fibers with elevated antibacterial and UV protection abilities [11].

Nanoparticles of other transition metals such as Fe, Cu, etc. were also tried for textile applications, but the stability of nanoparticles and their fixation on textiles had been hurdles in their utilization onto textiles [6].

Metal Oxides (TiO2 and ZnO)

Photocatalytic properties of titanium dioxide (TiO2) nanoparticles are highly utilized in textile industry for self-cleansing action of textiles. The fabrics are coated with a layer of TiO2 nanoparticles in which on exposure to light, photons with energy equal to or greater than the band gap of the titanium dioxide excite electrons up to the conduction band. These excited electrons produce free-radical oxygen on reacting with oxygen atoms in the air. High oxidizing power of these free radicals breaks down most of the carbon-based compounds in stains into CO2 and H2O through oxidation-reduction reactions [4]. One of these inorganic materials is zinc oxide (ZnO) and belongs to a group of metal oxides that are characterized for the properties such as photocatalytic ability, electrical conductivity, UV absorption, antimicrobial, and self-sterilization. Moreover, ZnO is generally regarded as a safe material for human beings and animals, and it has been used extensively in the formulation of personal care products for its characteristic properties such as nontoxicity, biosafety, excellent biological compatibility, high electron transfer rates, and enhanced analytical performance with increased sensitivity, ease of fabrication, and low cost of production [12]. ZnO is a wide-band-gap semiconductor and so has widely been used in mechanical actuators and piezoelectric sensors owing to their strong piezoelectric and pyroelectric properties. In recent years, ZnO has become an important material with potential applications in electronic and optoelectronic devices such as solar cells, field emission, displays, and sensors. Furthermore, ZnO is a versatile functional material that has diverse group morphologies [13]. Titanium dioxide (TiO2) nanoparticles in their two forms, which are anatase and rutile, were synthesized and applied onto cotton fabric’s surface by using ultrasonic irradiation. Antimicrobial activities of these nanoparticles on fabrics were evaluated against Escherichia coli, Staphylococcus aureus (bacteria), and Candida albicans (fungi). Appreciable results in terms of antimicrobial activity were observed primarily against Staphylococcus aureus, and the combination of these nanoparticles with visible light enhanced antimicrobial results [14]. Titanium dioxide (TiO2) nanoparticles had been studied for their self-cleansing activity along with the antimicrobial activity and UV protection ability in various studies [15,16,17]. Zinc oxide (ZnO) is actually one of the best bio-friendly absorbers of UV radiation and so highly utilized for their UV-shielding properties in textile industry. Synthesis of ZnO nanoparticles through a homogeneous phase reaction starting from zinc chloride and sodium hydroxide at high temperature, in water or in 1,2-ethanediol was reported for imparting UV-shielding effects on textiles. The UV tests indicated a significant increment to the UV-absorbing activity in ZnO-treated fabrics [18]. ZnO nanoparticles were also studied for their antimicrobial action on textiles [19, 20].

Nano-Composites

Nano-composites develop a new class of textile finishing materials which provide functionalities manifold relative to their smaller counterparts. In one of the studies Ag-ZnO nano-composites were prepared and applied on cotton and cotton/polyester blends to enhance the antibacterial activity of ZnO nanomaterials by Ag doping on their surface. Results were found highly influenced with the Ag doping. Ag nanomaterials are supposed to be more expensive in comparison to ZnO, so the little Ag doping enhanced activity of ZnO to great extent in form of Ag-ZnO nano-composites [21]. Sol-gel process at a low temperature was utilized to prepare anatase TiO2/SiO2 nano-composites and coated onto cotton fabrics by a simple dip-pad-dry-cure process. Treated cotton fabrics demonstrated higher photocatalytic activity for self-cleaning functionality in comparison to pure TiO2-treated cotton fabrics [22]. Besides composites of inorganic hybrids, polymer nano-composites were also studied for multifunctionalization of textiles. Polymer nano-composites with improved bonding affinities can be developed by using hydrophobic/hydrophilic functional polymer matrices as the dispersion medium for the nanoparticles [23].

Textile Materials (Wool, Silk, Cotton, etc.)

Textile fibers (Fig. 2) from both natural and synthetic origin are the primary structures for textiles which have their specific characteristic features in terms of strength and physical properties. These characteristics are based on the repeating units of monomers in polymeric structures of these fibers such as amino acids in wool and silk, cellulose in cotton, and caprolactam in nylon. Wool fibers are known for heat insulating, silk for anti-creasing, and cotton for its comfortness on the body. Some of regenerated cellulosic fibers (rayon, lyocell) and regenerated bamboo fibers were also produced which are semisynthetic in origin and contain cumulative properties of more than one natural fiber [24, 25].

Fig. 2
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Textile substrates for functional textiles

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Synthesis of Nanomaterials

Nanomaterials synthesis has been an important step in textile functionalization in terms of compatibility toward environment, precursors used, and the effluents or unutilized discards during the process. A lot of methods of “bottom-to-top approach” are utilized in research literature to get textile multifunctionalization via application of nanomaterials on them. Most of the chemical synthetic routes are associated with the use of hazardous chemicals for synthesis as well as stabilization of nanoparticles over textile surface.

A colloidal suspension of metal oxides or nanoparticles generated from a solution and then transformed into a gel through a thermal treatment, and finally into a dry network, is called the sol-gel process , a coating method, and can be applied on textile surface coatings also [26].

Light irradiation on to the textiles or polymer matrixes, which are previously treated in solution of metal precursors to be reduced into their nano-sized particles, is one of the methods to manufacture composite polymers containing nanoparticles. The method involves irradiating rays of light on the polymer matrix containing the metal precursor to reduce the metal precursor into uniformly distributed metal nanoparticles and fix the metal nanoparticles in the matrix or textiles [27]. Sonochemical irradiation is another effective method for the synthesis of nanomaterials and their deposition onto the textiles. This method utilized ultrasonic radiations, and advantages such as simple, efficient, one-step synthesis and production of uniform coating of metal nanoparticles on surfaces with different functional end groups were claimed. No significant damage to the yarn structure and stability on the fabric for washing were other associated advantages of the method [28]. A lot of research also evidences the use of plasma sputtering for the production and embedding of nanoparticles onto textiles for multifunctionalization [29, 30].

In situ synthesis of nanoparticles has also been evolved as an efficient method for the preparation and simultaneous functionalization of textile materials for various characteristics; particularly silver nanoparticles have been studied exhaustively for antimicrobial functionalization. Various reducing and stabilizing agents were used for simultaneous preparation and deposition of nanoparticles on textiles. In situ nano-silver coating of cotton fabrics via reduction of silver nitrate with glucose in the presence of cetyl trimethyl ammonium bromide (CTAB) as a stabilizer for excellent durable antimicrobial properties against S. aureus, E. coli, B. subtilis, and C. albicans was proposed by El-Shishtawy et al. (2011) [31]. Vigneshwaran et al. (2007) [32] reported a novel in situ synthesis protocol for silver nanoparticles onto cotton fabrics by immersing cotton fabrics into silver nitrate solution and autoclaved at 15 Ψ, 121 °C for 15 min. At these conditions, the aldehyde terminal of starch reduced the silver nitrate to silver metal and simultaneously stabilized them on fabric. 20.9 ± 13.7-nm-sized silver nanoparticle-impregnated cotton fabrics showed excellent antibacterial activity against bacteria S. aureus and bacteriostasis activity against K. pneumoniae. These fabrics also expressed significant UV protection abilities in comparison to that of untreated fabrics.

Biological compounds from natural resources particularly from plants were used to a great extent in nanomaterials synthesis and stabilization on textiles, owing to the abundant availability, biodegradability, and biocompatibility to environment and the living ecosystems. Caffeic acid as an eco-friendly reductant was employed for the one-step in situ synthesis and deposition of silver nanoparticles (AgNPs) on silk fabric. Silver nanoparticle-loaded silk fabrics were colored into golden yellow color and functionalized for high durability antibacterial effects [33].

Fabrication of Ag nanoparticle-loaded cotton fibers was developed by in situ synthesis of silver nanoparticles on cotton fabrics by extracts of Eucalyptus citriodora and Ficus benghalensis. Hydroxyl groups of extracts reduced and stabilized the silver nitrate into silver nanoparticles on cotton fibers. Surface hydroxyl groups on large surface area of cellulosic fibers can be used to facilitate the adsorption of silver nanoparticles quite efficiently onto cotton fibers. Developed Ag nanoparticle-loaded cotton fibers exhibited excellent antimicrobial activity against E. coli and suggestive of their utilization in wound dressings as well as in the fabrication of antibacterial textiles for biomedical applications [34].

Functional Nanomaterial Characterizations

Important characteristic feature of metal nanoparticles regarding textiles is the color production via localized surface plasmon resonance. Surface plasmon resonance produces characteristic peaks of maximum absorbance in UV-visible spectroscopy for various kinds of nanoparticles. This coloration property of nanomaterials is a different kind in comparison to conventional dyes. This is derived from the interaction of light with metal nanoparticles; conductive electrons oscillate locally around nanoparticles at a certain frequency, and the phenomenon of excitation of surface plasmons generated by light is called as localized surface plasmon resonance. Such optical property of nanoparticles is variable according to the shape and size of metal nanoparticles [10, 35].

When the nanoparticles are discussed, primary thing comes into mind is morphology (shape and size) of these synthesized nanoparticles. Electron microscopy (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) characterizations give the surface morphology and the size of synthesized nanoparticles. X-ray diffraction (XRD) patterns provide further information about the lattice arrangements. Specific characteristic peaks in XRD pattern are shown by the nanomaterials of different elements or elemental compositions (metal or composites). Nanoparticles deposition or the resulted nanomaterial-textile substrate composites are evaluated in terms of their substantivity of nanoparticles toward textiles. Starting of coloration of textiles (via surface plasmon resonance) in nanoparticles solution gives a primary clue about nanomaterial deposition. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) spectroscopy is the efficient technique to confirm the nanomaterials deposition on textile surfaces and also provides the elemental composition in nanomaterial-textile composites. Comparative analysis of XRD patterns and FT-IR (Fourier transform infrared) spectrum of textile substrates and nanomaterial-textile composites also provides related information or the interactions between nanoparticles and textiles. In some cases such as Ag nanoparticles, inductively coupled plasma optical emission spectrometry (ICP-OES) can also be helpful to determine the elemental content on fibers [31,32,33].

Applications Toward Textile Functionalization

Biomedical Textiles

Biomedical textiles are the most dynamically expanding sectors in the technical textile market. Antimicrobial textiles or surgical disposable medical textiles are the primarily focused textiles for biomedical applications. Antimicrobial and anti-inflammatory nature of silver nanoparticles is the most exploited nature of silver nanoparticles in the medical field highly used in wound dressings. Many of the mechanism of actions were described for antimicrobial nature of the nanoparticle-embedded textiles. But the exact mechanism by which nanoparticles cause antimicrobial effect is not clearly known till now. Most of the nanoparticles with large surface area have the ability to anchor to the bacterial cell wall and subsequently penetrate into it, result into permeability of the cell membrane, and lead to cell death [36]. Reactive oxygen species or free radicals formation via nanoparticles is also considered as one of the mechanism lead to cells death (Fig. 3). Electron spin resonance spectroscopy studies confirmed the formation of free radicals by nanoparticles in various studies [37, 38]. Another mechanism was based on the nature of some metal nanoparticles such as soft acid nature of silver, and its tendency to react with a soft base like sulfur and phosphorous bases of bacterial DNA results into destruction of DNA and ultimately cell death [39, 40].

Fig. 3
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Antimicrobial action mechanism of nanoparticles [36,37,38,39,40]

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UV-Protective Textiles

Exposure to ultraviolet (UV) radiations can lead to acute or chronic harmful effects on human life. These radiations are classified into three categories on the basis of their wavelength values. UV-A radiations age skin cells and can damage their DNA. These rays are linked to long-term skin damage such as wrinkles, but they are also thought to play a role in some skin cancers. Most tanning beds give off large amounts of UV-A, which has been found to increase skin cancer risk. UV-B radiations have more energy than UV-A radiations, and they can damage skin cells’ DNA directly and are the main radiations that cause sunburns. They are also thought to cause most skin cancers. UV-C rays have more energy than the other types of UV rays, but they don’t get through our atmosphere, as all are stopped by ozone layer in stratosphere. The acute effects of UV-A and UV-B exposure are both short-lived and reversible. These effects include mainly sunburn (or erythema) and tanning (or pigment darkening). The chronic effects of UV exposure can be much more serious, even life threatening, and include premature aging of the skin, suppression of the immune system, damage to the eyes, and skin cancer [18, 32].

There are two mechanisms for UV-protective behavior of textiles or sunscreens; one is via absorption of radiations and another via complete reflection of the radiations. Nanomaterials specifically opt the later via reflection of the radiations owing to the large surface area of them (Fig. 4). Nanomaterials used in textiles and sunscreens are beneficial because they have been found to reflect and scatter UV light, which is generally considered safer and more effective than absorption of UV radiations [41]. ZnO nanoparticles were synthesized and applied on cotton and cotton/polyester blended fabrics for UV protection functionalization and found shown high degree of UV protection [42].

Fig. 4
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UV protection via nanoparticles on textiles

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Flame Retardancy

Most of the textile fibers such as polyamide or polyester are inflammable and catch fire. A lot of studies had been carried out in past on the use of phosphorous containing and silica inorganic compounds for the application of flame retardancy [43, 44]. Last few years evidenced the use of nanomaterials also for this functionality on textiles. ZnO nanoparticles were applied on cellulosic fabrics with the help of cross-linking through different polycarboxylic acids with sodium hypophosphite as catalyst. Effect of concentration of sodium hypophosphite on flammability and UV protection is investigated. Increased flame retardancy of the treated cellulosic fabrics was observed successfully via ZnO nanomaterials [45]. ZnO nanoparticles have also been applied on sisal fiber yarn where the active enhancement of the property in nanoparticles as compared to bulk form has been scientifically analyzed. It was established that fire-retardant property of 1% ZnO nanoparticles is almost comparable to the 12% bulk ZnO on sisal fiber [46]. A new flame-retardant nano-coating has been constructed by the alternate adsorption of polyelectrolyte amino-functionalized multiwall carbon nano-tube (MWNT-NH2) and ammonium polyphosphate (APP) onto flexible and porous ramie fabric. Thermal and flammability property assessment for the pristine and nano-coated ramie fabrics shows that the thermal stability, flame retardancy, and residual char are enhanced with concentration of MWNT-NH2 suspension and number of deposition cycles increases. The enhancements are mostly attributed to the barrier effect of intumescent network structure, which is composed of MWNT-NH2 and the absorbed APP [47]. Improved flame-retardant properties of textile fabric polyethylene terephthalate (PET) using multilayered thin films were developed and evaluated via the application of silica nanoparticles using layer-by-layer assembly. Five bilayers of positively and negatively charged colloidal silica of <10 nm average thickness increased time to ignition and decreased heat release rate peak of PET fabric. Same nano-coating dramatically reduced burn time and eliminated melt dripping in vertical burn test. This study successfully demonstrated the ability to impart flame-retardant behavior using a water-based, environmentally friendly protective coating [48].

Hydrophobic Textiles (Self-Cleaning Property)

A lot of structures from nature are observed as super hydrophobic such as surfaces of the lotus leaf, rose petal, and duck feather surfaces. These structures are used as model surfaces to synthesize bio-mimic materials of hydrophobic properties with higher contact angle of water drops to these surfaces. Super hydrophobicity and water repellence of textiles can lead to the production of waterproof, self-cleaning, and stain-resistant clothing, so highly interested in textile industry [49]. Silica-based nanoparticles have been highly influenced researchers to achieve super-hydrophobic textile surfaces. Multifunctionalities in terms of hydrophobicity with simultaneous antibacterial and UV protection finishing were improved of cotton via coating prepared based on silica nanoparticles of 150 nm average particle size [50]. Super-hydrophobic surfaces via complex coating of amino- and epoxy-functionalized silica nanoparticles on epoxy-functionalized (enhanced interaction between fiber and silica coating) cotton textiles have been prepared. Lasting hydrophobic behavior was observed with the static water contact angle up to 170° (Fig. 5) for a 5 μL droplet [51]. In another study, super-hydrophobic cotton fabrics were successfully prepared via the incorporation of TiO2 particles by titania sol, and the coating not only caused a dual-size surface roughness for enhancing the hydrophobicity but also resulted into good UV-shielding ability [52].

Fig. 5
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Hydrophobicity in relation with contact angle of droplet with surface

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Conclusions and Future Outlook

A broad range of applications associated with the nanomaterials for their characteristic surface volume ratio make them fit in most of the aspects of life. Biomedical applications, drug delivery systems, UV protection sunscreens, and antimicrobial clothing are the applications achieved by the use of nanomaterials, that need higher concerns due to direct interaction of nanoparticles to human beings. The synthetic routes, stabilizing agents, and their release onto human system or environment are primary concerns to be focused, and a lot of such studies had been carried forward. Way of synthesis has significant effects on environment and biological ecosystem, as the use of precursors may produce sidewise hazardous products or their spill in to ecosystem. In case of nanomaterials, coloration comes via surface plasmon resonance, and UV protection abilities come via transmission of radiations on place of absorption in case of other UV-protective agents. These characteristics make them more prominent and modern resources for textile industry and hence exploration deserves. Primary concerns about use of nanomaterials are their binding affinities or substantivity toward textile surfaces. Natural polymers were also used as dispersion media for nanoparticles substantivity on textile surfaces. Textiles were also given various pretreatments to enhance substantivity in some of previous studies. Recently a number of studies have been carried out on utilization of natural extracts for reducing and stabilizing Ag ions on textiles which are quite successful efforts to minimize the environmental risk and human hazards. As the speed of development of nanomaterials and their wide applications in every aspect of life accelerating highly will lead to a better future in nanotechnological advancements. Now in the future, greenness of the processes of synthesis and stabilization of nanomaterials would be a target to develop green nanotechnology better compatible to the environment and human life.