SMART NANOMATERIALS *
CHAPTER 3 CHAPTER 3
3.6 CURRENT RESEARCH AND FUTURE PERSPECTIVES
3.6 CURRENT RESEARCH AND FUTURE PERSPECTIVES
The topic of “smart” nanotechnologies branches into a variety of fields, from nanomedicine to electron- ics. Important to many of these applications are core–shell responsive nanoparticles. It is thus impor- tant to understand their synthesis and responsiveness to external stimuli. Li et al. (2009) discussed different methods for preparing polymer-modified gold nanoparticles and their thermo-sensitivity and pH-responsiveness. The applications of such smart nanocomposites vary from: (1) nanocarriers for cat- alysts and biosensors (due to “live” outside polymer shells), (2) novel 2D/3D nanostructures for use in nanoelectronics, spintronics, and nanosensors (due to ability of self-assembly at surfaces or interfaces), and (3) intelligent pharmaceutical nanosystem with a controlled fashion for biological purposes. A direct synthesis yields nanocomposites with relatively good dispersity, long-term stability, and provides gold cores with a broad size distribution. Covalent “graft-to” strategy prepares gold nanoparticles with sulfur terminated polymers and yields low polymer graft density (due to the steric hindrance from large polymer chains). The “graft-from” strategy on the other hand provides controlled polymer molecular weight, narrow distribution, and shell thickness regularity.
According to Li et al.’s work, thermoresponsive materials, such as PNIPAM-b-PMOEGMA, show significant decrease in apparent Rh (due to contraction of polymer brushes) as the temperature increased;
2-phenylprop-terminated PNIPAM molecules at high temperature aggregate. Fig. 3.12 shows trans- mission electron microscopy (TEM) images of gold nanoparticles and polymer brushes in different pH solutions. It was found that for polymers such as PVP at higher pH levels, these brushes would contract and individual particles agglomerate. As pH decreases, these intramolecular forces decrease, spreading out the molecules and expending the particle’s polymer brushes. The study concluded that these smart nanocomposites comprising of intelligent polymers and gold nanoparticles can be prepared for different applications (each synthesis yields certain properties). The responsive polymer brushes undergo hydrophilic–hydrophobic phase transitions in response to stimulus such as temperature and pH differences.
FIGURE 3.12
TEM images at ph 3.1 (A), ph 4.4 (B), and ph 7.5 (c) in a detailed study of the pvp/gold nanoparticles.
From Li, D., He, Q., & Li, J. (2009). Smart core/shell nanocomposites: intelligent polymers modified gold nanoparticles. Advances in Colloid and Interface Science, 28–38.
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In the field of nanomedicine, drug delivery systems have been a central topic of study.
Biocompatibility of materials and innovative new ways of self-discharging drugs both hinder and allow for a wide range of mechanisms. There are a variety of mechanisms for drug delivery. Liu et al. (2009) studied the effect of particle size, “on-off” MF modes, and switching duration time (SDT) on ferrogels synthesized through freezing–thawing cycles combining poly(vinyl alcohol) hydrogel and magnetic particles (Fe3O4, Fig. 3.13A) of diameters ranging from larger particles (LM, 150–500 nm), middle size (MM, 40–60 nm), and smaller magnetic particles (SM, 5–10 nm). An MF was applied to PVA5-LM17, and it was observed that once ON, the swelling ratio of the ferrogel decreased (Fig. 3.13B, -▲-). This resulted in a decrease of drug release (Fig. 3.13B, -○-), since the particles contract and impede the dif- fusion of drug through the gel, “locking in” any drug within the magnetic particles. The permeability coefficient of PVA5-LM17 when the MF is “off” is 586 × 10−6 cm2/min. Once switched “on,” the coef- ficient decreases sharply to 40 × 10−6 cm2/min.
The SDT also affects the release profile of the drug, it was found that for a 10 min cycle, the “close”
configuration (which interlocks particles and reduces permeation of drug) was most effective in dif- fusing the drug once in the “off” mode and had a sharp decrease once returned to the “on” mode. LM particles were proven to be most suited for this experiment, since it presents the most “magnetic sensi- tive effects.” Smaller particles tended to aggregate together and therefore affected the “close” configu- ration. Ferrogels made of medium size particles had poor drug permeation. The study concluded that
“magnetic sensitive effects” are best observed in PVA5-LM17 ferrogels, because of superior saturation magnetization allowing strong MF induction, and smaller coercive force indicating ability of reorienta- tion of particles within the gel under an MF.
Much work is being done on innovative ways to improve or change drug delivery systems. Wu et al.
(2010) worked on a single nanoparticle that can integrate optical pH-sensing, cancer cell imaging, and controlled drug release. They discussed the characteristics and applications of an HPC–PAA–MBAAm
FIGURE 3.13
(A) cross-sectional SEM image of magnetic particles disperse in pvA hydrogels and OM photos of pvA5-LM17 ferrogels and (B) swelling ratio and swelling rate of pvA5-LM17 ferrogel in the Mfs switching “on–off” mode.
From Liu, T-Y., Hu, S-H., Liu, T-Y., et al. (2006). Magnetic-sensitive behavior of intelligent ferrogels for controlled release of drug. Langmuir.
63 3.6 cuRRENT RESEARch AND fuTuRE pERSpEcTIvES
nanogel developed by the group with in-situ immobilization of CdSe quantum dots (QDs). The study examined the gel’s pH-sensing, tumor cellular imaging, and pH-regulated drug release. Each element of the gel served a purpose: cadmium selenium QDs were used for optical identification codes for sens- ing and imaging, HPC chains’-OH groups prevented agglomeration and release of QDs, PAA chains provided pH sensitivity and allowed for delivery systems via swelling/shrinking transition, and car- boxyl groups were used for bioconjugation and potential targeting ability. This hybrid nanogel is able to emit different fluorescence wavelengths, which is beneficial for sensor and bioimaging applications.
Its ability for carrying drugs (such as TMZ) and high drug loading capacity was studied under differ- ent pH stimuli and was found to be very effective. Fig. 3.14 shows the two template nanogels (without the 3.2 nm QDs). Denser cross-links in HNG-2 resulted in smaller swelling degree under different pH solutions affecting drug release properties of the hybrid gel.
Throughout their experiment the stability of the nanogel was studied and even at the most swollen state, QDs were not released from the nanogels due to the strong binding. The study concluded that the developed hybrid nanogel shows excellent ability as a drug carrier due to its porous nature and swell- ing abilities, providing opportunities for combined diagnosis and therapy ability (switch on and off of nanogel functions). Also, the addition of phosphorescent QDs enhances optical features, which are beneficial for sensor and bioimaging applications.
As discussed in Chapter 4, Nanosensors, sensors are an important part of nanotechnology. The use of smart nanomaterials can be incorporated into the field of strain sensors, as illustrated by Kuilla et al.’s (2010) study on the use of graphene-based piezoresistive smart nanomaterials. Kuilla et al. examined graphene-based nanohybrid material and its piezoresistive characteristic in order to develop smart novel materials for potential application in graphene strain sensors. A graphene/epoxy composite was fabricated
FIGURE 3.14
TEM images of hpc/AA/MBAAm template stable nanogels (A) the hNG-1 (40:10:10 wt%) and (B) hNG-2 (40:15:20 wt%) Rh<100 nm.
From Wu, W., Aiello, M., Zhou, T., Berliner, A., Banerjee, P., Zhou, S. (2010). In-situ immobilization of quantum dots in polysaccharide-based nanogels for integration of optical pH-sensing, tumor cell imaging, and drug delivery. Biomaterials.
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via casting mold. Graphene was directly distributed into an aqueous epoxy polymer without the use of a solvent via an ultrasonic homogenizer for 2 h. Jeffamine polytheramines was added in order to induce curing. The subsequent mixture of graphene, epoxy, and jeffamine was placed in a vacuum oven at 50°C and 70 cm Hg for 10 min. The grapheme/epoxy composite was then poured into a silicon mold and cured at room temperature for 24 h. After removing the mold, the cast composite was cut into the desired sizes.
In order to fabricate a sensor, two electrodes were connected onto the cast composite with silver conduct- ing epoxy. The sensor was attached to a steel beam. The piezoresistance of the sensor was measured via a multimeter. The beam deflection was measured with a laser displacement sensor to find the sensor defor- mation. A cantilever beam was used to model and test the sensor. The beam displacements and the sub- sequent sensor resistance were measured. The sensor results were compared with composite CNTs strain sensors. The strain response of graphene/epoxy sensor was found to be symmetrical and exhibit reversible behavior. The gauge factor obtained was approximately 11.4 and within a range of 1000 microstrain. The graphene composites showed higher gauge factor than strain gauge made of high-quality graphene film.
Furthermore, the strain sensitivities of graphene composites were found to be much higher than CNT composites. It was concluded that higher strain sensitivity of graphene composites may have been due to the larger inter-contact areas among the graphene nanofillers due to their 2D structure.
One of the emerging applications of smart nanomaterials is in the development of smart nanotex- tiles. The synthesis of these smart textiles often requires the use of sophisticated techniques. One such technique was outlined in the research conducted by Kinloch, Li, and Windle (2004). Kinloch et al.
examined the spinning of CNT fibers and ribbons directly from the chemical vapor deposition (CVD) synthesis zone of a furnace using a liquid source of carbon and an iron nanocatalyst. In this process, liquid ethanol (carbon source) was used in which 0.23–2.3 wt% ferrocene and 1–4 wt% thiophene were dissolved. The solution was then injected from the top of a furnace into a hydrogen carrier gas. The furnace temperature was maintained between 1050°C and 1200°C. The high temperature vaporized the ethanol/ferrocene/thiophene complex and the process underwent CVD to form an aerogel of nano- tubes. The aerogel is captured and wound out of the hot zone continuously as a fiber by a rotating spindle. The alignment, purity, and structure of the fibers obtained from the ethanol-based reactions were characterized by electron microscopy, including image analysis, Raman spectroscopy, and ther- mogravimetric analysis. The quality of alignment of the nanotubes was measured from transforms of scanning electron microscope (SEM) images. The formation of MWNT or SWNT can be controlled by adjusting the reaction conditions. If the thiophene in the ethanol feedstock is adjusted to between 1.5 wt% and 4.0 wt% and the reactor temperature is between 1100°C and 1180°C MWNTs are formed, while if the thiophene concentration is reduced to 0.5 wt% with a reaction temperature of up to 1200°C SWNTs are formed. It was observed that the purity of the CNTs fibers were directly proportionate to the hydrogen flow rate. High hydrogen flow rates were found to suppress the formation of other forms of carbon impurities. When analyzing the MWNT fibers, the nanotubes diameters were 30 nm, with an aspect ratio of ~1000. They contained 5–10 wt% iron but no other carbon contaminants. It was observed that the degree of alignment can be improved if greater tension is applied to the fiber during processing. The SWNT fibers contained more impurities than the MWNT fibers, with the proportion of SWNTs estimated from transmission electron microscope observations as being more than 50 vol.%.
The SWNTs had diameters between 1.6 and 3.5 nm and were organized in bundles with lateral dimen- sions of 30 nm. The purity of MWNTs in fibers spun at high temperatures was higher (85–95 wt%
purity) than for material collected from the furnace without spinning (70–85 wt% purity) (Fig. 3.15).
As previously mentioned, one of the criteria of smart material is their ability to react to changes in their surrounding environment. There has been a growing interest in synthesizing smart nanomembranes
65 3.6 cuRRENT RESEARch AND fuTuRE pERSpEcTIvES
with the ability to change their permeability when subject to external stimuli. Such material can have great potential in medicine in the case of drug delivery systems. The ability to synthesize such smart mem- branes was researched and the subsequent results were published by Csetneki et al. (2006). Csetneki et al.
examine novel composite-gel membranes containing nanochannels that are capable of regulating mem- brane permeability in response to external temperature change. The channels contain an ordered array of magnetic polystyrene latex particles that undergo change in volume in response to external stimuli. The magnetic polystyrene latex was prepared by mixing ferrofluids of magnetite (Fe3O4) particles with an aver- age diameter of 10 nm with a mixture of styrene, sodium lauryl sulfate, stearyl alcohol, and N,Nʹ-azobis (isobutyronitrile). Magnetic polymer latex was prepared by the miniemulsion technique and was allowed to polymerize. The latexes were subsequently subjected to water-vapor distillation and washed to remove the unreacted monomers. N-isopropylacrylamide monomer, methylene bis acrylamide as the cross-linker, and potassium persulfate (KPS) as the initiator were mixed with the magnetic polystyrene latex. The shell polymerized under a nitrogen atmosphere resulting in particles of core–shell MPS–PNIPAM microgel latex with a thermosensitive PNIPAM surface layer. The membrane was fabricated by mixing the MPS–
PNIPAM microgel latex with a solution of polyvinyl alcohol (PVA) containing glutaric aldehyde as cross- linking agent. The cross-linking reaction of PVA was induced by adding a few drops of hydrochloric acid.
The mixture was transferred into a mold and subsequently placed perpendicularly to a uniform MF. The resulting reaction locked and aligned the chainlike structure in the gel along the direction of the MF. The core–shell MPS–PNIPAM particles formed channels in the PVA matrix and when subject to temperature change began to swell. It was concluded that as the temperature surrounding the membrane changed, the permeability drastically changed. With increasing temperature the permeability increased. Below the critical temperature (37°C) the channels in the PVA membranes were saturated with MPS–PNIPAM latex beads and as a result the solutes permeability was limited. The polymer chains restrict the mobility of the FIGURE 3.15
(Right) Depiction of cNT fiber being drawn on rotating spindle. (Left) A video frame view up the furnace, showing the nanotubes being drawn from the aerogel into the fiber on the spindle.
From Kinloch, I., Li, Y., & Windle, A. (2004). Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis.
Science Magazine, 276–278.
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solute molecules, resulting in a reduced diffusion coefficient. When the temperature was increased above the critical transition temperature (37°C), the size of the MPS–PNIPAM latex beads reduce causing the bead diameter to be less than that of the channel, the MPS–PNIPAM beads no longer filled up the entire channels, and polymer free cavities were formed. The open cavities allowed for an increase in solute per- meability and an increase in diffusivity (Fig. 3.16).