Review on synthesis, properties, characterization, and applications of responsive microgels fabricated with gold nanostructures

2.1 Hybrid microspheres

Au nanostructures are distributed in the whole polymer network in hybrid microspheres. Cross-linking density is uniform in the whole microgel particle (Table 1). Nanostructures are fabricated in the sieves of microgel particles. Hybrid microspheres swell/deswell abruptly because the distribution of monomers and nanostructures is uniform throughout the microgel particle. Gorelikov et al. (2004) and Shi et al. (2014) have reported the synthesis of hybrid microspheres for drug delivery and catalytic applications, respectively.

2.2 Hybrid microgel rings

Au nanostructures are uniformly fabricated in a ring-shaped microgel network. This hybrid system has gained attention, because ring thickness is very small and nanostructures come on surface in shrunken state easily. Lange et al. (2012) have synthesized Au-poly(N-isopropylacrylamide) [Au-p(NIPAM)] hybrid microgel rings in which Au nanoparticles are present in the p(NIPAM) network. Hollow space at the center is filled with solvent, so both the inner and outer surfaces of the ring are in full contact with the solvent. Moreover, the hollow space can act as a storehouse for drugs and other materials. These materials can be transported in large amount to target place via hybrid microgel rings. That is why this architecture of hybrid microgels is considered the most suitable for drug delivery.

2.3 Core-shell hybrid microgels

The structure of core is different from that of shell in these microgels. This is a versatile class of hybrid microgels. Various architectures of core-shell hybrid microgels have been synthesized by scientists whose detail is as follows.

1. Au nanostructure can act as core and cross-linked network can act as shell to form core-shell hybrid microgels (Budhlall et al. 2008). Center acts as plasmonic material and shell modulates the value of SPR wavelength (λ_SPR) by swelling/deswelling (Kim and Lee 2007). Kim and Lee (2006) have synthesized core-shell hybrid microgels for drug delivery; they used Au nanoparticle as core and synthesized poly(NIPAM-co-acrylic acid) [p(NIPAM-co-AAc)] microgels as shell. The catalytic activity of nanoparticles can be effectively controlled using this type of core-shell hybrid microgels because the catalyst is present at the center and the diffusion of reactants is controlled by shell. Carregal-Romero et al. (2010) have synthesized Au-p(NIPAM) core-shell hybrid microgels for catalytic applications. They controlled diffusion of reactants towards Au nanoparticle by modulating the cross-linking of p(NIPAM) shell. Shiotani et al. (2007) used Au nanorods as core and p(NIPAM) network as shell for photothermal drug delivery.

2. Nanostructures are present at the periphery of these microgel particles. Therefore, microgel acts as core and nanostructures act as shell. Kumar et al. (2007) have synthesized such core-shell microgels in which p(NIPAM) microgel particle is surrounded by Au nanorods. These hybrid microgels are most applicable in the field of photonics because nanostructures are present at the periphery only. Thus, the distance between nanostructures in swollen and deswollen state can be easily calculated.

3. Core and shell both are made up of cross-linked polymeric network, but Au nanostructures are uniformly fabricated in shell only. Suzuki and Kawaguchi (2005a,b) have synthesized such hybrid microgels. Poly(glycidyl methacrylate) [p(GMA)] acts as core and Au-p(NIPAM) acts as shell. Such hybrid microgels have not been used for any application yet, but they can be used in sensing and photonics.

4. Core and shell of this type are composed of cross-linked network, but Au nanostructures are present as layer at the core-shell interface only. Suzuki and Kawaguchi (2005a,b) have also synthesized these hybrid microgels. They synthesized poly(NIPAM-co-glycidyl methacrylate) [p(NIPAM-co-GMA)] microgels, then they used these microgels as seeds for the fabrication of p(NIPAM) shell, and then they fabricated Au nanoparticles at the core-shell interface.

2.4 Core-shell-shell hybrid microgels

The structure of core-shell-shell hybrid microgels is a modified form of core-shell hybrid microgels. When another layer of the polymer network is present around core-shell hybrid microgels, then core-shell-shell hybrid microgels are formed. All three layers are composed of polymeric network. These hybrid microgels are not suitable for photothermal drug delivery because nanoparticles are not distributed in the whole microgel particle. These hybrid microgels are suitable for catalysis and surface enhanced sensing. Suzuki et al. (2007) have prepared core-shell-shell hybrid microgels in which Au nanoparticles are fabricated in the middle layer only. Agrawal et al. (2013) also synthesized core-shell-shell hybrid microgels in which Au nanoparticles were present in core only and then they used these hybrid microgels for catalytic applications.

2.5 Yolk-shell hybrid microgels

This is also a modified form of core-shell hybrid microgels. In such kind of microgels, a space is present between core and shell. Core is composed of Au nanostructure and shell is composed of the polymer network. This architecture of hybrid microgel is most suitable for catalytic applications because the space around nanoparticle provides a homogenous environment of reagents for catalysis. The diffusion of reagents can be easily controlled with the help of microgel shell. Moreover catalyst dosage can be effectively controlled with yolk-shell architecture because one nanoparticle is present per hybrid microgel particle. Wu et al. (2012) have prepared Au-p(NIPAM) yolk-shell hybrid microgels for the catalytic reduction of 4-nitrophenol (4-NP) and nitrobenzene (NB). They also explained that these hybrid microgels are the best candidate for catalysis because reactants are present in the space between nanoparticles and polymeric network. Therefore, reactants are in full contact with nanoparticles and catalysis is done in an effective manner. Lange et al. (2012) have also synthesized Au-p(NIPAM) yolk-shell hybrid structures, but the shell thickness of their microgels is very small compared to those reported by Wu et al. (2012).

3.1 Synthesis of microgels

Polymer network Au-microgels are synthesized by free radical precipitation and inverse emulsion polymerizations. Negative and positive temperature-responsive polymers are synthesized by precipitation and inverse emulsion polymerization, respectively (Echeverria and Mijangos 2010, Shi et al. 2013).

Precipitation polymerization for the synthesis of temperature-responsive microgels is carried out at a temperature greater than the volume phase transition temperature (VPTT) of the microgels in aqueous medium. If NIPAM is monomer, then the reaction mixture is heated up to 70°C in aqueous medium (Zhang et al. 2012). Monomers, surfactant, and cross-linker are mixed initially. Then, initiator (e.g. ammonium persulfate) is added. Initiator gives negatively charged free radicals. Free radicals have the ability to react with oxygen of air and reagents of the reaction mixture. The possibility of consumption of radicals by oxygen has terminated because the reaction is carried out under nitrogen atmosphere. Therefore, the radicals react with monomers and cross-linker and fabricate a cross-linked polymer network. The microgel dispersion is cooled and dialyzed using macromolecular porous membrane tubing. Dialysis is carried out for 7 days against distilled water at room temperature. Unreacted monomers are washed out from microgel dispersion by dialysis. Nearly monodisperse microgel particles are synthesized using this polymerization (Suzuki et al. 2007). The yield of product (microgel) depends on reaction time duration. A large amount of product (microgel) has been formed in initial first hour, but reaction is continued further only to maximize the amount of product.

Sometimes, microgels are coupled with some moieties that increase the scope of applications of microgels. For example, Shi et al. (2013) synthesized p(NIPAM-co-AAc) microgels and then they coupled carboxyl groups of AAc with 2-aminoethane thiol (AET) via 1-ethyl-3-[3-(dimethylamino)-propyl] carbodiimide hydrochloride. Coupling-modified microgels possess thiol at the tip of pendant group of AAc units. The authors then used these microgels as microreactors for the in vivo synthesis of Au nanoparticles. Then, the length of pendant group has increased after coupling. Therefore, it becomes easier to hold nanoparticle in place than that of noncoupled microgels. That is why the authors claimed that the AET pendant group has increased the stability of nanoparticles compared to that of carboxyl group. Similarly, Zhang et al. (2012) coupled p(NIPAM-co-AAc) microgels with 3-acrylamidophenylboronic acid (APBA) to make them glucose responsive. They also claimed that APBA has increased the stability of nanorods and the scope of this system for biomedical applications.

The morphology of microgel particles and distribution of monomer units inside the polymer network depends on the time of addition of monomers into reaction vessel. If all reagents are initially added, then they were incorporated homogenously in the polymer network. Gorelikov et al. (2004) have synthesized p(NIPAM-co-AAc) microgels by adding all monomers before the initiation of polymerization. If one monomer is added at one time and other is added later, then core is formed by monomer added first and shell is formed by monomer added later. Agrawal et al. (2013) added N-vinylcaprolactam (VCL), acetoacetoxyethyl methacrylate (AAEM), sodium dodecyl sulfate, and N,N′-methylene-bis-acrylamide (BIS) and initiated polymerization by adding initiator. After some time, they added AAc into reaction mixture. Thus, AAc resides in shell of the particles. They confirmed the core-shell structure of microgels by transmission electron microscopy (TEM) images. It is better to incorporate a thermoresponsive monomer at core and pH-responsive monomer at shell because the pH-responsive monomer causes greater swelling than that of thermoresponsive monomer. In this way, swelling due to the pH-responsive network is not hindered.

Microgels are also synthesized by inverse emulsion polymerization. For this purpose, emulsifier is added into organic phase. Monomers and cross-linker are dissolved in aqueous phase. Then, the aqueous phase is added dropwise into organic phase with constant stirring. The reaction mixture is heated up to a certain temperature depending on the monomers used. After some time, initiator is added. Initiator then initiated polymerization and turbidity is visible in the flask. Later, microgel dispersion is cooled and dialyzed against fresh water to remove unreacted reagents and organic phase. In this way, positive temperature-responsive microgels are synthesized by inverse emulsion polymerization. Echeverria and Mijangos (2010) have synthesized Au-poly(acrylamide-co-AAc) [Au-p(AAm-co-AAc)] core-shell hybrid microgels using this method.

3.2 Three approaches towards synthesis of Au-microgels

Au-microgels can be synthesized using the following methods.

4.1 Temperature-responsive behavior

Microgels swell and deswell in response to the increase in temperature of the medium depending on the nature of the polymer network (Hou and Wu 2014). When Au nanostructures are loaded within microgels, then their temperature sensitivity does not vanish (Dulle et al. 2015). Hybrid microgels show temperature sensitivity similar to that of pure microgels. However, the VPTT of hybrid microgels is found to be different from that of pure microgels (Khan and Alhoshan 2013).

Based on the nature of temperature-responsive monomer, the responsive behavior of Au-microgels is classified as negative and positive temperature-responsive behaviors. The temperature-responsive behavior of both types of hybrid microgels has been discussed here in detail.

4.2 pH-responsive behavior

The microgels having ionic units inside the network are known as pH-responsive microgels. Hybrid microgels are pH responsive, similar to that of pure microgels, because the fabrication of Au nanostructures does not distort their pH-responsive behavior. Based on the nature of ionic monomer, there are three types of pH-responsive Au-microgels: cationic, anionic, and ampholyte hybrid microgels.

4.3 Photoresponsive behavior

The photoresponsive behavior is the property of a hybrid microgel to swell and deswell in response to light. Au nanostructures possess SPR band. They may absorb in visible, microwave, or infrared regions depending on the size of nanostructures and environment around nanostructures. When the wavelength of incident radiation approaches the λ_SPR, then resonance occurs and nanostructures absorb those radiations. Au nanostructures absorb light energy due to SPR and convert it into heat energy through radiationless relaxation process. This heat energy causes VPT in hybrid microgels. Shiotani et al. (2007), Budhlall et al. (2008), and Suzuki et al. (2007) have synthesized hybrid microgels that are responsive to infrared, microwave, and visible radiations, respectively. They used these hybrid microgels in drug delivery, microlensing, and glucose sensing. Photothermal swelling and deswelling is reversible. Scientists have reported that Au-microgels can deswell and swell on many heating and cooling cycles (Budhlall et al. 2008). Au-p(NIPAM-co-acrylamide) hybrid microgels were eight times exposed to microwave radiations for 5 min. The hybrid microgel showed shrinking on exposure to radiation and swelling in the absence of radiation. They observed that the photothermal swelling/deswelling of hybrid microgel was reversible. It was also observed that Au nanoparticles did not coalesce or leak out from microgels during consecutive heating and cooling cycles.

4.4 Glucose-responsive behavior

Glucose-responsive microgels swell and deswell in response to change in glucose concentration. These microgels possess the glucose-responsive moiety as pendant group. When Au nanoparticles or nanorods are incorporated within these microgels, then their glucose sensitivity does not vanish. Hybrid microgels show glucose responsive behavior similar to that of pure microgels. Boronic acid is a glucose-responsive moiety. Acidic carboxyl groups of microgels are coupled with APBA to produce glucose-responsive microgels. When the pH of the medium is increased and reached above the pK_a of boronic acid, then boronic acid changes into boronate ions. Boronate ions have high tendency to bind with glucose compared to boronic acid. When boronate ions bind with glucose molecules, then the polymer network is dragged outside due to this new bond formation. Therefore, water molecules rush into hybrid microgels and swelling occurs. The number of bonded glucose molecules increases with the increase in the concentration of glucose. The size of hybrid microgels increases by increasing the concentration of glucose up to a certain limit. When all boronate ions have bonded to glucose molecules, then the size of microgels is not affected by the increasing concentration of glucose anymore. Zhang et al. (2012) have synthesized glucose-responsive poly(N-isoproylacrylamide-co-3-acrylamidophenylboronic acid) [p(NIPAM-co-APBA)] microgels and Au-p(NIPAM-co-APBA) hybrid microgels. They studied their glucose sensitivity by increasing the glucose concentration from 0 to 30 mm. They observed that the size of microgels and hybrid microgels was linearly increased by increasing the glucose concentration from 0 to 10 mm. However, the size of microgels and hybrid microgels are not affected by increasing the glucose concentration from 10 to 30 mm. They concluded that 10 mm is the threshold value after which microgels did not show glucose-responsive behavior. When the concentration of glucose is 10 mm in the medium, then all boronate ions have bonded to glucose, so the size of microgels did not change between 10 and 30 mm glucose concentration. They observed that pure and hybrid microgels show glucose sensitivity between 0 and 10 mm. Therefore, the incorporation of Au nanorods did not affect glucose sensitivity range of microgels. Glucose-responsive hybrid microgels are largely used in insulin delivery devices and glucose sensors due to their glucose-responsive behavior.

8.1 Biomedicine

Microgels fabricated with Au nanorods have been used in photothermal drug delivery and photothermal therapy fields of biomedicine. Hybrid microgels transfer drug into body by swelling/deswelling of responsive polymeric network. The longitudinal SPR band of Au nanorods lies in the near-infrared range. Therefore, Au nanorods absorb near-infrared radiation and transfer energy to microgels. This energy increases the temperature of microgels; hence, microgels deswell and solvent molecules move out. Near-infrared radiation is not harmful for the human body, because cells of the body do not absorb in this region. Thus, drug loaded within hybrid microgels can be easily transferred to medium on photothermal deswelling. Any carrier of nanorods that fulfills the following criteria can be used for drug delivery. Fortunately, microgels fulfill all the following criteria.

1. Temperature- and pH-induced swelling/deswelling must take place at physiological conditions (~37°C temperature and ~7.45 pH). p(NIPAM-co-AAc) microgels fulfill this demand (Das et al. 2007).

2. The swelling ratio of carrier must be large, so the maximum number of drug molecules can be transferred via a small dose of carrier. The swelling ratio of all microgels is very large. Even the swelling ratio of microgels can be controlled by varying cross-linking density. Thus, the extent of release of drug can be controlled using microgels (Das et al. 2007).

3. The nanorods should remain stable upon reversible swelling and deswelling within carrier. Microgels also fulfill this demand. Nanorods are found to be stable within microgels. They do not aggregate or leak out from microgels upon consecutive swelling and deswelling cycles (Gorelikov et al. 2004, Shiotani et al. 2007).

Gorelikov et al. (2004) loaded Au nanorods into p(NIPAM-co-AAc) microgels and designed a photoresponsive system for drug delivery (Figure 1). They optimized the SPR band of nanorods in the near-infrared range by changing the aspect ratio of nanorods. They found that nanorods of aspect ratio 3 possess SPR band at 804 nm at pH 4. Therefore, they bombarded hybrid microgels with laser of 810 nm wavelength. This photothermal heating induced deswelling in these hybrid microgels and water molecules moved out. If drug-loaded hybrid microgels are implanted into the body and the targeted area of body is bombarded by 810 nm radiation, then drug molecules can also be moved out as a result of photothermal deswelling. Das et al. (2007) and Kawano et al. (2009) have also synthesized such Au-microgels, which showed deswelling upon exposure to near-infrared radiation. Das et al. (2007), Kawano et al. (2009), and Gorelikov et al. (2004) practically did not load any drug into hybrid microgels. They did not study whether hybrid microgels worked after the loading of drug or not. This gap was fulfilled by Shiotani et al. (2007). Shiotani et al. (2007) loaded rhodamine-labeled dextran drug into Au-p(NIPAM) core-shell hybrid microgels. They used near-infrared laser of power 750 mW and wavelength 1064 nm. They observed that the drug was really moved out of hybrid microgels due to photothermal deswelling. They confirmed it using TEM images and dynamic light scattering (DLS) measurements. Rhodamine is a fluorescent drug, so they confirmed the position of the drug in the system from fluorescence spectroscopic images. It is clear from these images that drug molecules reside within hybrid microgels in swollen state while they move out in deswollen state. They had also taken images after consecutive heating and cooling cycles. They observed that, once the drug had been delivered, then the drug molecules did not move back into microgels on cooling. Therefore, their work gives confirmation to the research of Gorelikov et al. (2004), Das et al. (2007), and Kawano et al. (2009).

Figure 1:

Photothermal drug release from microgels fabricated with Au nanorods (adapted from Gorelikov et al. 2004).

Au-microgels absorb radiation and transfer their energy to medium in the form of heat. In this way, they increase the temperature of the medium. The viability of tumor cells is affected by temperature. Hybrid microgels have been used to kill tumor cells by photothermal therapy. Zhang et al. (2014) have used Au-p(NIPAM-co-AAc) hybrid microgels for the in vivo treatment of breast cancer 4T1 cells by photothermal therapy. Initially, they optimized the size of hybrid microgels in swollen and deswollen states, so that microgels could easily travel through tumor vessels. They also studied that cell viability was not affected by the presence of p(NIPAM-co-AAc) microgels. Then, they implanted hybrid microgels into tumor cells and then irradiated that body part with laser having wavelength comparable to the longitudinal λ_SPR of nanorod and observed tumor weight loss. They also heated the affected body part by dipping mice in hot water and observed tumor weight loss. They observed that weight loss in tumor cells by photothermal therapy is greater than that by thermal therapy. Doxorubicin drug is used for the chemical treatment of 4T1 cells. They compared tumor weight loss by doxorubicin-loaded hybrid microgels and naked doxorubicin. They observed that tumor weight loss in both cases is almost same. However, they observed that hybrid microgels delivered drug into nucleus of cells, whereas naked doxorubicin could not approach the nucleus of cells. Thus, hybrid microgels effectively transferred drug into cells. They also compared the photochemical and thermochemical treatment of tumor cells by drug-loaded hybrid microgels. They observed that the tumor inhibition rate by photochemical treatment was faster than that by thermochemical treatment. In this way, they practically studied photothermal therapy and drug delivery by hybrid microgels.

8.2 Sensing

8.2.1 Microgel-based sensing

APBA-coupled hybrid microgels are well-known glucose-responsive materials. Zhang et al. (2012) used APBA to design a glucose sensor that can detect glucose at low concentrations. They coupled APBA with p(NIPAM-co-AAc) microgels to prepare p(NIPAM-co-APBA) microgels. Then, they mixed microgel dispersion with Au nanorod dispersion and synthesized hybrid microgels. They mixed both dispersions at different pH. They optimized the conditions to such limit at which nanorods show the maximum shift in the λ_SPR in response to a very small change in the concentration of glucose. They observed that synthesized hybrid microgels swell in the presence of glucose. Due to swelling, the λ_SPR of nanorods shifted and the signal is recorded due to the shifting of the λ_SPR. They observed that the λ_SPR decreases from 723 to 672 nm when the concentration of glucose was increased from 0 to 10 mm at pH 9 and temperature at 30°C. When the concentration of glucose was increased above 10 mm, then no change in the λ_SPR and the size of microgel were observed. It means that this sensor could measure the concentration of dilute glucose solutions only. Zhang et al. (2012) did not mention that synthesized hybrid microgels can be used for the delivery of insulin within the body. Hybrid microgels swell in response to glucose concentration, so insulin can be easily leaked out from microgels in swollen state. Insulin decreases the level of glucose in human body. Insulin is not produced by the body of diabetic patients, so insulin injections are given to them. If insulin-loaded hybrid microgels are injected into their bodies, then there is no need to check their blood glucose level from time to time. The APBA of microgels automatically checks the blood glucose level and releases insulin according to the demand of the body. A pictorial diagram of glucose concentration-mediated drug release is shown in Figure 2.

Figure 2:

Insulin delivery by glucose concentration-mediated swelling/deswelling in microgels fabricated with Au nanorods (Zhang et al. 2012).

8.3 Catalysis

Au-microgels are largely used in the catalysis of different chemical reactions. A pictorial diagram of the catalytic applications of various Au-microgels are shown in Figure 3. Hybrid microgels have the following advantageous properties that facilitate the process of catalysis.

Figure 3:

Catalytic applications of various kinds of Au-microgels.

1. The high swelling ratio of hybrid microgels facilitates the diffusion of reactants towards the surface of nanoparticles.

2. Hybrid microgels are responsive to various stimuli, so they help to tune the catalytic activity of nanoparticles by various stimuli, such as temperature and pH of the medium.

3. Hybrid microgels can be easily separated from the reaction mixture by centrifugation or ultrafiltration. Therefore, microgel-based catalyst can be used again and again.

4. The catalytic activity of hybrid microgels can also be tuned by varying structural parameters of microgels, such as cross-linking density.

5. Microgels resist the leaching of nanostructures. Therefore, hybrid microgel-based catalysts do not lose their catalytic activity and can be reused.

Shi et al. (2013) used Au-p(NIPAM-co-AET) hybrid microgels as catalyst for the reduction of 4-NP in aqueous medium. They studied the effect of temperature on the value of apparent rate constant (k_app) of reduction. NIPAM is a thermoresponsive monomer. Therefore, they observed a nonlinear dependence of ln k_app on 1/T (Figure 4). Initially, the value of ln k_app increases by increasing the temperature from 30°C to 36°C due to the Arrhenius behavior of reaction. Then, the value of ln k_app decreases by increasing the temperature from 36°C to 40°C. This is due to the deswelling of hybrid microgels. Due to deswelling, the approach of reactants towards the surface of catalyst becomes difficult and the value of ln k_app decreases. Then, the value of ln k_app again increases by increasing the temperature from 40°C to 52°C. This is again due to Arrhenius behavior. Agrawal et al. (2013) studied the effect of the surface area of catalyst on k_app of 4-NP reduction using Au-poly(N-vinylcaprolactam)-poly(aceto acetoxy ethyl methacrylate)-poly(AAc) [Au-p(VCL)-p(AAEM)-p-(AAc)] core-shell-shell hybrid microgels as catalyst. They prepared three different hybrid microgels in which the size of Au nanoparticles was same but the number of nanoparticles was different. They observed that the value of k_app is maximum for the sample having the highest number of nanoparticles among all samples. Actually, the surface area for catalysis increases by increasing the number of nanoparticles; hence, the value of k_app increases by increasing the surface area. They also observed a very small decrease in the catalytic activity of hybrid microgels when they were reused again and again. Wu et al. (2012) studied the catalytic reduction of hydrophilic 4-NP and hydrophobic NB reduction within the same medium at same time using Au-p(NIPAM) yolk-shell hybrid microgels. They observed that the value of k_app of the reduction of 4-NP is greater than that of NB before the VPTT. They also observed that the value of k_app of the reduction of 4-NP is smaller than that of NB after the VPTT. Actually, NIPAM is hydrophilic before the VPTT and hydrophobic after the VPTT. Also, 4-NP is comparatively more polar compared to that of NB. Like dissolves like, so 4-NP rapidly diffuse through the p(NIPAM) network before the VPTT and NB rapidly diffuse through polymer network after the VPTT. They also observed nonlinear dependence of ln k_app on 1/T due to the responsive nature of hybrid microgels.

Figure 4:

Temperature-dependent catalytic activity of Au-p(NIPAM) hybrid microgels (conditions: [4-NP]/[NaBH₄]=400 and Au-p(NIPAM)=0.06 mg/ml; Shi et al. 2013).

The reduction of potassium hexacyanoferrate (III) (K₃[Fe(CN)₆]) was studied by Carregal-Romero et al. (2010) using Au-p(NIPAM) core-shell hybrid microgels as catalyst. They studied the effect of catalyst dosage, temperature and cross-linking density of polymer network on the catalytic activity of hybrid microgels. They observed that the value of k_app linearly increases by increasing the catalyst dosage. They also observed that the catalytic activity of swollen hybrid microgels is different from that of deswollen hybrid microgels. The difference between the catalytic activity of hybrid microgels in swollen and deswollen states depends on their swelling ratio because nanoparticles of exterior portion are only available in the deswollen state. The swelling ratio of hybrid microgels decreases by increasing the cross-linking density. It means that densely cross-linked hybrid microgels are less temperature responsive compared to that of loosely cross-linked hybrid microgels. Therefore, the catalytic activity of densely cross-linked microgels was not significantly affected by temperature and the catalytic activity of loosely cross-linked microgels was significantly affected by temperature.

Jia et al. (2015) compared the catalytic activity of α-cyclodextrin-modified Au-p(VCL) hybrid microgels for the reduction of 4-NP and 2,6-dimethyl-4-nitrophenol (2-DMNP). They noticed that the catalytic activity of Au-p(VCL) hybrid microgels was greater for 4-NP in comparison to 2-DMNP. They explained that α-cyclodextrin formed strong complex with 4-NP and weak complex with 2-DMNP. That is why the difference in catalytic activity is observed due to the difference in substrate-catalyst affinity.

This discussion reveals that Au-microgels are applicable for the catalysis of organic and inorganic reactions. Scientists proved that the catalytic activity is dependent on catalyst dosage, size of nanoparticles, structure of microgels, and temperature. However, they did not calculate the energy, entropy, and enthalpy of activation of catalytic process in the swollen and deswollen states of hybrid microgels. From Figure 4, it is clear that the slope of plot in swollen and deswollen states are different from each other. It means that activation parameters are different in swollen and deswollen states, so they must be reported. Moreover, the actual mechanism of catalysis has not been discussed in detail yet. NIPAM-based microgels are soluble in organic solvents at high temperatures, but Au nanoparticle-based hybrid microgels have not been used as catalyst in nonaqueous medium. Therefore, some gaps are still present in the study of catalytic applications of Au-microgels, and more work is necessary in this area to explain the hidden facts in the field of catalysis by Au-microgels.

8.4 Photonics

Au-microgels have the ability to diffract light rays. Photothermal swelling and deswelling occur in them. Deswollen microgel particles can assemble into glassy and crystalline phases as discussed in Section 6. The distance between nanostructures decreases due to deswelling in microgels. The sieves of microgels are of nanosize, so the distance among nanostructures approaches the diffraction limit in deswollen microgels. Therefore, deswollen hybrid microgels can act as diffraction grating. When visible radiation falls on microgel-based grating, then radiation diffracts into rings. Thus, light rays are diffracted into a specific direction using microgel-based grating. This property of hybrid microgels was used by Jones et al. (2003) to prepare photothermally patterned microlens. Jones et al. (2003) used Au-p(NIPAM) hybrid microgels as a microlens to magnify the fine details of images. When hybrid microgels are irradiated with a wavelength of 532 nm, then hybrid microgels deswelled rapidly and converted into crystalline ordered phase. They placed an object on one side of hybrid microgels and a visible radiation source on the other side of hybrid microgels. When radiation falls on crystalline phase, then they diffract towards object. They illuminated the object and magnified its fine details. Words that were not clearly visible by hand lens become visible by microlens due to diffraction. They explained that diffraction occurred due to the reflection of light rays from the microgel-air interface (A) and mirror-air interface (A′). d is the distance between microgel-air interface and mirror-air interface. A path difference of λ/2 is induced between ray A and A′ (Figure 5). These rays constructively interfere at this path difference; hence, illumination and magnification of image occurred. Thus, hybrid microgel-based crystalline phase can be used as an efficient microlens in place of expensive quartz crystals.

Figure 5:

Illumination and magnification of an image by Au-p(NIPAM) hybrid microgels (Jones et al. 2003).

8.5 Coating

Microgels help in the coating of nanostructures in the form of thin film on different substrates. Initially, hybrid microgel particles are dispersed on water in a trough. A substrate, such as glass slide, is present in trough in tilted orientation (i.e. making 45° angle with the base of trough). Then, water is drain out from trough, so that water level starts decreasing. Microgel particles also move down due to the downward movement of water. Microgel particles are smoothly adsorbed on substrate due to the downward movement of water. Then, the pressure is applied for strong adherence of hybrid microgels on the substrate. Then, the substrate is annealed at high temperature or exposed to plasma, so that the microgel network gets decomposed. Nanostructures will be naked and coated on substrate. Yolk-shell and core-shell hybrid microgels are most suitable for this purpose because one nanostructure is present per microgel particle. Therefore, the chances of heterogeneous coating can be reduced. Vogel et al. (2012) have practically coated glass slide with Au nanoparticles with the help of Au-p(NIPAM) yolk-shell structures. They studied that the distance between nanostructures are not affected by the magnitude of applied pressure. However, the magnitude of applied pressure affected the degree of order in coating. Long-range ordered coating was obtained at high magnitude of applied pressure. Clara-Rahola et al. (2014) have studied the effect of temperature on the coating of Au nanoparticles. As R_h of Au-p(NIPAM) core-shell hybrid microgels decreases with the increase in temperature, the distance between nanoparticles in microgel film decreases with the increase in temperature. That is why they coated hybrid microgels on glass slide at different temperatures. As a result, they observed that the number of nanoparticles per unit area of substrate increased with the increase in temperature. Nanostructure-coated substrates have found applications in SERS ultrasensitive analysis and photonics (Vogel et al. 2012). According to the best of our knowledge, no one has practically used such nanostructure-coated substrates for any application. Hence, more work is needed in this area to extend it towards the applied direction.