One-step synthesis of platinum nanoparticles loaded in alginate bubbles
© Yang et al.; licensee Springer. 2014
Received: 11 April 2014
Accepted: 24 May 2014
Published: 30 May 2014
Composite particles with multifunctions have been extensively utilized for various applications. Bubble particles can be applied for ultrasound-mediated imaging, drug delivery, absorbers, cell culture, etc. This study proposes a one-step strategy to obtain Pt nanoparticles loaded in alginate bubbles. A needle-based droplet formation was used to generate uniform alginate particles about 2 mm in diameter. The hydrolysis reaction of NaBH4 was utilized to produce gaseous hydrogen and then trapped within alginate particles to form bubbles. The Pt4+ mixed with alginate solution was dropped into the reservoir to react with reducing NaBH4 and hardening CaCl2 to form Pt nanoparticles-alginate composite bubbles. Results indicate that the size of bubbles decreases with the CaCl2 concentration (1% ~ 20%), and size of bubbles increases with the NaBH4 concentration (1 ~ 20 mM). The advantages for the present approach include low cost, easy operation, and effective production of Pt nanoparticles-alginate composite bubbles.
The metal nanoparticles (NPs) are powerful products of nanotechnology, providing broad variety of applications in life science [1, 2]. For example, drug delivery, cellular imaging, and biosensing have been extensively described [3–6]. The chemical versatility of metal NPs holds the potential to outclass in a number of applications . These unique properties and applications of metal NPs are well reviewed [7–9]. Platinum is used in various applications such as catalysts in many organic reactions [10, 11], preparation of organic dyes , and biomedical applications [13, 14]. For example, the Pt NPs were employed for successful photothermal treatment of Neuro 2A cancer cell by using irradiation with 1,064 nm near-infrared pulse wave and the Nd YAG laser set at 3 W for 480 s. The Pt NPs increased 9°C in temperature leading to effective photothermal killing of cancer cells .
The Pt composite materials have gained much attention due to their good multifunctions [16, 17]. Pt NPs-chitosan composite particles have been extensively studied over the last decade [18, 19], and Pt NPs-chitosan composite bubbles are one of the most emerging and intriguing topics [20, 21]. Bubble particles have import features entrapping air bubbles inside. Due to their low density, bubble particles can float on liquid surface for specific applications. They can also be applied as novel vehicles for ultrasound-mediated imaging and targeted drug delivery followed by burst release [22–27]. Besides, bubble particles can be utilized as absorbers to facilitate adsorption of substrates due to a high-surface area. Pt NPs-chitosan composite bubbles can be applied in controlled release and tissue engineering; however, chitosan carrier substrates will disintegrate and dissolve in acid solution such as gastric juices. Therefore, Pt NPs-chitosan composite bubbles are limited in acidic condition. Fortunately, alginate polymer provides a solution to overcome this problem. Alginate polymer has a dense structure to pass the acid solution. To our best knowledge, Pt NPs-alginate composite (Pt NPs@alginate) bubbles are seldom reported in literatures, and they can provide applications for wide pH ranges.
By extending our previous works to prepare uniform alginate particles [28–31] and alginate bubbles , this work further develops a novel one-step method to fabricate composite Pt NPs@alginate bubbles through a simple chemical reaction. The Pt NPs and bubbles within alginate particles are investigated and characterized. The manufactured alginate products will provide great promise for multifunctional applications.
Alginic acid sodium salt (Na-alginate, brown algae with viscosities 150 cp and 350 cp in 2% (w/v) solution at 25°C) and dihydrogen hexachloroplatinate (IV) hexahydrate, ACS, Premion, 99.95% were obtained from Alfa Aesar (Johnson Matthey Company, London, UK). Sodium borohydride (NaBH4) was purchased from Sigma (Sigma Chemical Co., St. Louis, MO, USA), and calcium chloride (CaCl2) was obtained from J.T. Baker (J.T. Baker Chemical Company, Phillipsburg, NJ, USA). All chemicals and solvents were of analytical reagent grade.
Mechanism of bubbles formation
The NaBH4 hydrolysis is spontaneous, and gaseous H2 generation continues with the hydrolysis reaction. Due to the density difference, generated H2 bubbles move upwards in the reservoir solution. After a dropwise addition of Na-alginate solution into the reservoir, gas bubbles were entrapped within alginate particles to be alginate bubbles. One alginate particle can hold many numbers of bubbles by random. After 30 min, alginate bubbles were collected by filter, washed twice with 30 mL dd-H2O, and finally collected and characterized.
Preparation of Pt NPs@alginate bubbles
An optical microscope system (TE2000U, Nikon, Lewisville, TX, USA) and a USB digital microscope (UPG621, UPMOST Technology Corp., Taipei, Taiwan) were utilized to observe the morphology of the collected particles. To minimize selection bias, a total of more than 50 individual particles were analyzed to ensure statistical representation. X-ray diffraction (XRD, D2 Phaser, Bruker AXS Gmbh, Germany) patterns were obtained at room temperature by using Cu K-α radiation (30 kV/10 mA) with a range of 2θ = 20° ~ 80°, and a scanning rate of 4° min−1. Laser Raman spectroscopy was obtained using a Renishaw Microscope Raman Spectrometer (Renishaw plc., Gloucestershire, UK) from 200 to 1,100 cm−1 at room temperature. The 785-nm line of the laser was used as the excitation source, with the capability of supplying 300 mW. The morphology of the particle composites was analyzed using a scanning electron microscope (SEM, S-3400, Hitachi Ltd, Tokyo, Japan) and a transmission electron microscope (TEM, FEI Tecnai G2 20 S-Twin; FEI Company, Hillsboro, OR, USA) equipped with a METEK (PV 97–56700 ME) X-ray energy dispersive spectrometer (METEK Meteorologische Messtechnik GmbH, Elmshorn, Germany).
Cell viability test
The viability of the control and the treated cells were evaluated using 3-(4,5-cimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay with human breast adenocarcinoma MCF-7 cells (1 × 104/well) seeded in a 96-well microtiter plate with a 100 μL culture medium treated with various amounts of Pt NPs@alginate bubbles. After 1 day exposure, a 200-μL MTT solution was added to react with the cells for 4 h. After removal of the medium, 100 μL DMSO was added and examined at 595 nm using a microplate reader (Multiskan Ascent, Thermo Electron Corporation, Vantaa, Finland). The control group in the untreated well was considered to be 100%.
Results and discussion
Pt NPs@alginate bubbles
Alginate is a kind of polysaccharide from marine brown algae. A variety of fundamental properties such as excellent biodegradability and biocompatibility make alginate a very attractive material for applications. Alginate has been applied in diverse areas [34–36] including serving biomedical materials for drug delivery and tissue engineering, and being adsorbent materials for elimination of heavy metals or organic pollutants . Due to acid dissolution, conventional Pt NPs@chitosan bubbles have constraint applications for limited pH conditions. Therefore, it is needed to develop Pt NPs@alginate bubbles for wide pH applications.
Reduction reaction of Pt salts by reducing agents such as borohydrides and citrates is one of the convenient methods to prepare Pt NPs . This study demonstrates a proof-of-concept approach for encapsulating the Pt NPs and bubbles into alginate particles utilizing simple reduction and hydrolysis reactions. Produced Pt NPs@alginate bubbles combined the characteristics of Pt NPs and bubbles. The composite bubble particles can provide wide applications, such as smart vehicles for ultrasound-mediated imaging and targeted drug delivery, and effective absorbers and catalysts for decomposing pollutants. In the future, this proposed strategy to formulate Pt NPs@alginate bubbles can also be applied for synthesis of other composite materials.
Particle morphology of chitosan and alginate immersed in different solutions
This paper developed a facile method to synthesize platinum nanoparticles within alginate bubbles. Sodium borohydrate was utilized to generate platinum NPs and gaseous hydrogen by reduction reaction and hydrolysis reaction, respectively. Bubbles entrapped within around 2-mm alginate particles increased with the borohydrate concentration and alginate viscosity. This proposed one-step method to prepare Pt NPs@alginate bubbles has advantages of low cost, easy operation, and effective pore formation. Compared with conventional Pt NPs@chitosan bubbles, Pt NPs@alginate bubbles provide more applications for wide pH ranges.
This work was financially supported by a grant from the Ministry of Science and Technology of Taiwan, Republic of China.
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