107. Chen YC, Hsiao JK, Liu HM, Lai IY, Yao M, Hsu SC, Ko BS, Chen YC, Yang CS, Huang DM. The inhibitory effect of superparamagnetic iron oxide nanoparticle (Ferucarbotran) on osteogenic differentiation and its signaling mechanism in human mesenchymal stem cells. 2010;245:272-9
Ellington . used this bioconjugation method to conjugate an anti-EGFR aptamer onto GNPs to demonstrate the targeting specificity of a newly identified aptamer . As described above, they coated GNPs with thiol-modified capture ONTs that hybridized to an extended anti-EGFR aptamer to generate anti-EGFR aptamer-conjugated GNPs. Jon . utilized the hybridization method to conjugate an anti-PSMA-aptamer to iron oxide nanoparticle surfaces . In their study, CG-rich duplexes were constructed at the ends of the aptamers to achieve multiple Dox-binding on the nanoparticles. They immobilized amine-functionalized capture ONTs (5'NH2-A10-(TCG)7-3') on the surfaces of the carboxyl-modified iron oxide nanoparticles. Magnet purification yielded the capture ONT-coated iron oxide nanoparticles. A (CGA)7-extended anti-PSMA-aptamer was synthesized by transcription, and the aptamers were added to the ONT-coated nanoparticles to produce Apt--TCL-SPIONs. Thermal gravimetric analysis (TGA) indicated that PSMA aptamers (28,451 Da) hybridized to approximately 33% of the ONTs (9,708 Da) conjugated to the nanoparticles. aptamers capture ONTshe surface of nanoparticles via staining in larger samples (106 cells).ons [nitude highenn
Anisotropic gold nanoparticles, namely gold nanorods, Au-silica core-shell nanomaterials and ultra-small gold nanoparticles were synthesized by Duraiswamy and Khan in segmented flow operation in PDMS-made (Polydimethyloxysiloxane) microchannels . They demonstrated that precise and rapid micromixing within droplets furnished high quality GNRs. Three separate streams containing (1) preformed gold seeds denoted as S (3+, Ag+ and CTAB (R1) and (3) ascorbic acid solution (R2) were premixed in a micromixer and then fed into the second micromixer where Taylor flow was created by addition of a silicone oil stream ( panel: 1 and 2). Due to intense recirculation inside each droplet, the growth of seeds into rods occurred in a reproducible way, which in turn produced monodisperse GNRs, with insignificant amount of by-products (irregular colloidal gold). By manipulating silver content in the droplets, GNRs of different aspect ratios were produced (; panel 3). The reactor was used for 12 h without any channel fouling.
In GNRs synthesis, reliable and continuous supply of stabilized ultra-small gold nanoparticles (or seeds) is of importance to realize a reliable production facility. As discussed in the first part of this review, several authors attempted microfluidic synthesis of ultra-small GNPs using a strong reducing agent, sodium borohydride. However, sodium borohydride is known to decompose in water and produce hydrogen gas bubbles. Generation of additional gas phase in single phase operation may enhance convective mixing between liquid elements, however in a rather irregular manner . Such unpredictable and irregular mixing between liquid reactants would make the process unreliable for sustained and reproducible synthesis. Saif . suggested to use gas-liquid flow in which nitrogen bubbles were introduced to absorb hydrogen released from the sodium borohydride solution (A,B) . In this system, hydrogen gas forming in the aqueous phase rapidly diffuse into the intervening nitrogen bubbles due to large concentration gradient and intense vortex in the liquid slugs. Thereby, hydrogen concentration in the aqueous phase was kept well below the saturation point to suppress nucleation and micro bubbles formation inside the aqueous slugs. A diffusion model was developed which showed that the hydrogen concentration in liquid remained below the solubility threshold of 0.9 mM. In that way, the process was run for 8 h. Spherical gold nanocrystals of below 5 nm in size were produced (C). These seeds were successfully used to produce high quality GNRs in batch synthesis. This work envisages that a full continuous process of seed formation and subsequent use in GNRs synthesis would be possible without any flow mal-distribution by NaBH4 decomposition.
Ruoslahti . described self-amplifying tumor homing nanoparticles . The system was based on a CREKA peptide that not only recognizes clotted plasma proteins around tumor vessel walls or tumor stroma but also induces localized tumor clotting [-]. Fluorescein-labeled peptides, including the sulfhydryl group of the single cysteine residue, were coupled to amino dextran-coated iron oxide nanoparticles (CREKA-SPIO), and nanoparticles with at least 8,000 peptide molecules per particle were used for experiments. To reduce reticuloendothelial system (RES) uptake, a major obstacle to the homing of the nanoparticles, chelated Ni2+-liposome or liposomes as potential decoy particles were introduced prior to CREKA-SPIO injection. CREKA-SPIO treatment after pretreatment with the decoy particles displayed primary localization in the tumor vessels, and fewer particles were seen in the liver. The tumor-targeted nanoparticles were distributed along a meshwork in the clots, presumably formed by fibrin, suggesting that the nanoparticles infiltrated the depths of the clots. The tumor magnetization was quantitatively analyzed using a superconducting quantum interference device (SQUID), revealing that heparin injection prior to injection of CREKA-SPIO reduced the tumor accumulation of nanoparticles by >50% by eliminating intravascular clotting, although the treatment series did not considerably reduce the number of vessels. Thus, binding of CREKA-SPIO to tumor vessels did not require clotting activity, but intravascular clotting attracted more nanoparticles to the tumor, suggesting that tumor targeting was amplified.
Typically, chemistry of GNRs formation involves templated growth of small gold seeds (1–3 nm) with a growth solution: a mixture of gold salt, ascorbic acid reductant, templating agent CTAB and a small amount of silver nitrate in water. Boleininger . reported the first GNRs synthesis in a PVC microreactor connected to a three-way valve and a 7-port manifold (Upchurch Scientific) . The authors were able to produce GNRs with different aspect ratio by changing the ratio between the preformed gold seeds solution and the growth solution. In-situ monitoring of the GNRs formation was performed with an optical probe in the downstream part of the microreactor. The effect of the reaction temperature and reactant concentrations was systematically studied. Since preformed seeds for GNRs are not stable over a period of several hours, only freshly prepared seeds can serve as reliable core materials for their growth into monodisperse GNRs. In this way, Bullen . modified this synthesis by employing a sequentially rotating tube processor (RTP) connected to a microfluidic chip to perform sequential operations of seed formation and growth of seeds into GNRs () . A stable continuous operation for 19 days was demonstrated, which would be hampered had they used presynthesized seeds. Spectral data for all the samples collected over 19 days of operation demonstrate the robustness of the system to produce high quality of gold nanorods having reproducible quality and least by-products (spherical or irregular-shaped gold nanoparticles) as evident from the TEM picture.
Although many synthetic routes have been developed for the preparation of iron oxide core with tunable shape, size and magnetization, several challenges remain for the naked SPIONs in terms of stem cell labeling, including: (i) poor water solubility and tendency of aggregation due to large surface/volume ratio; (ii) low cellular uptake efficiency; (iii) potential toxicity. To address these problems, the most straightforward and effective method seems to be coating the iron oxide core by a layer. The nature of the surface coatings and modification methods determine the physical and biologic properties such as the overall size, surface charge, coating density, toxicity and degradability, which finally affect the fate of SPIONPs in the cells [, ]. This following section focuses on the currently used surface modification materials (e.g. PLL, PEI, chitosan, PEG, citric acid and so on) and methods (e.g. coating, post-synthesis coatings including blending, polymerization, ligand exchange) for the SPIONs applied for stem cell labeling and tracking. The influence of these factors on labeling efficiency and biocompatibility is also discussed.
In the “seedless” method, a gold precursor solution (HAuCl4, CTAB, acetylacetone) and a reactant mixture (CTAB, pH 10 carbonate buffer and AgNO3) were fed into the RTP (30 cm × 6 cm, rotating at 1000 rpm). The centrifugal force of the RTP generated a dynamic thin film (300 µm) on the inner wall of the reactor facilitating the Au-seed formation in 30 s. These seeds subsequently grew to nanorods of 24.2 nm × 6.6 nm in the microfluidic chip. Typically, the control of shape and size of anisotropic gold nanoparticles is accomplished by adjusting the concentration of shape modulating agents: silver and CTAB capping agents. By varying the silver concentrations in the second feed, the authors were able to control the aspect ratio of the produced rods. Although, such capping agents enable synthesis of high quality GNRs, both Ag and CTAB exhibit cytotoxicity, limiting their direct use in biological systems.
So far, we considered syntheses under single-phase conditions, via controlled mixing and reactions between two or more miscible reagent solutions. This protocol is simple and easily adaptable to different microfluidic set-ups. However, there are several drawbacks associated with single-phase materials synthesis: (1) due to axial diffusion in laminar flow, there exists wide distribution in the residence time (A,C), which contributes to polydisperse and intractable mixture of several products and by products [,]; (2) mixing occurs solely by diffusion, which requires micromachining of additional structural elements (e.g., pin-fin or split-and-recombine) with lithographic techniques to induce convective flux to assist mixing (Hydrodynamics and reaction studies in a layered herringbone channel ; (3) deposition of gold nanoparticles occurs onto the inner reactor walls. The deposit serves as nucleation sites hence depleting feed stock in an unproductive way. The reactor becomes unusable at longer reaction times, which impedes the utility of the reactor for long-term use. Passivation of the channel surface with hydrophobic functional groups was proposed as a possible remedy; however, such methods are not generic for all materials used for microfluidic device fabrication. Specifically, while PDMS is the most widely used materials for microfluidic reactor fabrication, unfortunately PDMS is unsuitable for such chemical passivation.
In many multiphase operations, reagents containing aqueous streams are pumped inside the microfluidic mixing port/junction and are segmented into small droplets or slugs by introducing an immiscible, non-reacting fluid (gas or oil) . Such droplets/slugs translate through the channel. Due to internal circulation inside these liquid segments, intense inter-phase mixing becomes possible in addition to molecular diffusion. Hence, excellent mixing is accomplished in the segmented flow, which produces a very narrow residence time distribution profile. Moreover, droplets/slugs are produced in high frequency and fidelity in terms of their content concentrations, volume where all droplets act as identical reaction flasks, which are essential to reduce polydispersity and in many cases require minimal/no post-purification operation. All these beneficial attributes of multi-phase microfluidics enabled synthesis of nanoparticles of superior quality and wide chemical-structural diversity [,,,]. In the following section, we discuss recent advances in multiphase microfluidics for gold nanoparticles synthesis.