Silica Nanoparticles for Micro-Particle Imaging Velocimetry - American

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Silica Nanoparticles for Micro-Particle Imaging Velocimetry: Fluorosurfactant Improves Nanoparticle Stability and Brightness of Immobilized Iridium(III) Complexes David J. Lewis,†,∥ Valentina Dore,‡,∥ Nicola J. Rogers,†,∥ Thomas K. Mole,† Gerard B. Nash,§ Panagiota Angeli,*,‡ and Zoe Pikramenou*,† †

School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom Chemical Engineering Department, University College London, Torrington Place, London, WC1E 7JE, United Kingdom § School of Clinical and Experimental Medicine, Medical School, University of Birmingham, Birmingham B15 2TT, United Kingdom ‡

S Supporting Information *

ABSTRACT: To establish highly luminescent nanoparticles for monitoring fluid flows, we examined the preparation of silica nanoparticles based on immobilization of a cyclometalated iridium(III) complex and an examination of the photophysical studies provided a good insight into the Ir(III) microenvironment in order to reveal the most suitable silica nanoparticles for micro particle imaging velocimetry (μ-PIV) studies. Iridium complexes covalently incorporated at the surface of preformed silica nanoparticles, [Ir-4]@ Si500-Z, using a fluorinated polymer during their preparation, demonstrated better stability than those without the polymer, [Ir-4]@Si500, as well as an increase in steady state photoluminescence intensity (and therefore particle brightness) and lifetimes which are increased by 7-fold compared with nanoparticles with the same metal complex attached covalently throughout their core, [Ir-4]⊂Si500. Screening of the nanoparticles in fluid flows using epi-luminescence microscopy also confirm that the brightest, and therefore most suitable particles for microparticle imaging velocimetry (μ-PIV) measurements are those with the Ir(III) complex immobilized at the surface with fluorosurfactant, that is [Ir-4]@Si500-Z. μ-PIV studies demonstrate the suitability of these nanoparticles as nanotracers in microchannels.

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INTRODUCTION The miniaturization of chemical systems offers many advantages including continuous processing, improved yields or purity, and reduction of side-products and of waste.1−3 Microfluidic devices incorporating novel channel architectures encourage mixing in a passive manner.4,5 The increased surface to volume ratio means that interfacial phenomena such as Marangoni and Taylor flows become significant.6 Such microchannels can also be used as high-throughput analytical platforms for a range of interesting targets7−9 or as devices to extract certain molecules from solution.10 Particle imaging velocimetry (PIV) is a popular image analysis technique used to extract Eulerian velocity fields by dividing a flow domain into windows of interrogation and using pairs of images to produce a displacement vector from a cross-correlation routine.11,12 With the advent of micro-PIV (μ-PIV),13 smaller particles are required to investigate microchannels. Nanoparticles present a unique opportunity to exploit a bottom-up approach to materials fabrication to produce tracers for μ-PIV. We have been interested in strategies for attaching luminescent probes in nanoparticles, using 13 nm gold nanoparticles as a scaffold on which to assemble metal complexes,14 for their delivery and imaging into cells both selectively15 and nonselectively.16 We recently employed luminescent silica nanoparticles17 based on their wider availability in sizes that can be tracked with high accuracy. Metal complexes incorporated into silica nano© 2013 American Chemical Society

particles display far superior stability to that of organic dyes as fluorophores in μ-PIV experiments,18 thus conferring the benefit of an increased shelf life and solution recycling. Iridium(III) polypyridyl complexes, especially cyclometalated complexes, have excellent photophysical properties within the set of d6 metals and often display high quantum yields.19 The bright luminescence from iridium(III) complexes has been exploited for in vitro imaging applications20 as well as incorporation into materials for organic light-emitting diodes (OLEDS).21,22 While the incorporation of ruthenium(II) complexes into silica is fairly well explored,23−29 reports of the incorporation of cyclometalated iridium(III) species, which in general are more hydrophobic, into silica are relatively few, by either covalent18,30 or noncovalent means,31−33 although supramolecular switches have been used to both entrap and release an iridium(III) complex in mesoporous silica using redox, light, pH changes, or electrochemical stimuli.34−37 To develop brightly luminescent nanoparticles with metal complexes for μ-PIV applications an evaluation of the photophysical properties with the mode of metal complex attachment is highly desirable, to avoid issues with leakage of the lumophore that are unavoidable in simple adsorption-based Received: August 16, 2013 Revised: September 30, 2013 Published: October 28, 2013 14701

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Scheme 1. Synthesis of Ir-1 and Ir-4, and Routes to Nanoparticle Incorporation to Give [Ir-1]⊂Si500, [Ir-4]⊂Si500, and [Ir-4] @Si500-Z

isolated by centrifugation at 6000g and the supernatant removed, and redispersed by sonication into fresh ethanol (30 mL) and the process repeated. The particles were then centrifuge-washed with water (2 × 30 mL) and dried under vacuum in a desiccator containing calcium chloride at room temperature to leave a pale yellow solid. [Ir-4]@Si500-Z. To a 0.05% w/v suspension of [Ir-4]@Si500 (5 mg) in water (10 mL) was added Zonyl 7950 fluorosurfactant (CH3CH2CCOOC2H4(CF2)nCF3, MW ca. 500, 1.15 g mL−1, 80 μL). The colloid was immersed in an ultrasonic bath for 30 s and used directly in imaging experiments, or diluted further with deionized water to 0.004% w/v for ζ-potential and photoluminescence (PL) spectroscopy measurements. [Ir-4]⊂Si500. To a vigorously stirred mixture of tetraethylorthosilicate (1.3 mL), water (3 mL), ethanol (23.7 mL) and a preprepared 0.025 M solution of Ir-4 in DMSO (0.100 mL) was added aqueous ammonia solution (30% w/w, 7 mL). The mixture rapidly became turbid, and was stirred for 1 h, followed by sonication for 10 min. Particles were purified by centrifugation and redispersion into ethanol (3 × 30 mL) and water (1 × 30 mL). [Ir-1]⊂Si500. To a stirred mixture of tetraethylorthosilicate (1.3 mL) in ethanol (5 mL) was added a solution of aqueous ammonia

preparation routes such as [Ir-1]⊂Si500 (Scheme 1). In this article, we report the incorporation of cyclometalated iridium(III) complexes into nanoscale silica by surface coating with a cyclometalated Ir-4 complex and a fluorosurfactant to yield [Ir4]@Si500-Z (Scheme 1). The photopysical studies are compared with silica nanoparticles with the same complex incorporated covalently throughout the particle, [Ir-4]⊂Si500, and with a similar Ir-1 complex adsorbed in silica, [Ir-1]⊂Si500. We show unequivocally that [Ir-4]@Si500-Z are the brightest particles for imaging applications and showcase their use for constructing velocity fields by μ-PIV in a microchannel.

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EXPERIMENTAL SECTION

Nanoparticle Preparation. [Ir-4]@Si500. To a vigorously stirred mixture of tetraethylorthosilicate (1.3 mL) and water (3 mL) in ethanol (18.7 mL) was added aqueous ammonia solution (30% w/w, 7 mL). The mixture rapidly became turbid, and was stirred for 2 h, after which time a preprepared 0.025 M solution of Ir-4 in DMSO (0.25 mL) was added. The sol was immersed in an ultrasonic bath for 2 min and left to stir at room temperature for 24 h. Silica particles were 14702

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(30% w/w, 7 mL) in ethanol (18.7 mL), followed by Ir-1 (0.5 mg mL−1) in acetonitrile (3 mL). The particles were stirred for 1 h and then purified by centrifugation-redispersion into water (3 × 30 mL) and then ethanol (1 × 30 mL) and dried over calcium chloride at room temperature in a vacuum desiccator to leave a white solid. Flow Imaging. [Ir-1]⊂Si500, [Ir-4]⊂Si500, and [Ir-4]@Si500-Z silica particles were imaged in the same microchannel used for photophysical measurements and detailed above. The flow speed was controlled using a Harvard Instruments syringe pump set at 5 μL min−1. The observation plane is located approximately 1 μm away from the channel wall along the z-axis. Due to limitation in the working distance of the objective, it was not possible to focus further away from the channel wall. The location of the observation plane was determined by using a precision traverse calibrated using the thickness of the channel wall close to the observation plane and further verified by calculating the location of the focal plane of the optics given the objective specifications. A set of ca. 900 images was acquired at 20 Hz (50 ms of integration time). Images were preprocessed to enhance particle contrast against the background noise. The background was then subtracted from each raw image. The background image was generated based on the local minimum intensity over the image acquisition. In this way, bright static objects (i.e., channel walls) are removed while the background noise is significantly reduced. PIV analysis was performed using Insight 3G (TSI) to resolve velocity components within an equally spaced grid drawn on the planar domain. Each image was sampled into overlapped interrogation windows. The displacement of each window across two subsequent frames was then reconstructed via a cross-correlation routine. Pixel displacements were converted to velocities by using the pixel size (i.e., 400 nm) and time-lag between two correlation frames (i.e., 50 ms). The calibration of the images was performed by using the width of the channel as reference distance. By performing a standard PIV routine, instantaneous planar velocity fields can then be resolved within the domain. In this work, flows were steady-state, and it was possible to calculate a correlation based ensemble-averaged PIV over the entire image acquisition.38 The ensemble PIV, by virtually increasing the seeding density within the interrogation windows, leads to more robust velocity statistics. A square domain discretization of 32 × 64 pixels with an overlap of 50% guaranteed both robust statistics within each interrogation window and a good spatial resolution along both the axial and transverse direction of the flow (6.4 μm × 12.8 μm). The depth of correlation (2zcorr, Olsen and Adrian)39 is 0.87 μm, which is remarkably narrow mainly as a consequence of the small particle size. For further reading on the μ-PIV technique, the reader is referred to works by Lindken et al.,13 Rossi et al.,40 Cierpka and Kähler,41 and Gui and Werely.42

of the reagents and generally leads to well-defined spherical morphologies. Furthermore, using the chemistry developed by van Blaaderen and co-workers, it is moderately easy to adapt the synthesis either to incorporate organic ligands onto the surface of the particles postsynthetically or to modify the interior of the particles using organosilane functional groups, like those in Ir-4.46 We were able to graft Ir-4 onto the surface of the preformed silica nanoparticles by hydrolysis of the two integral silanes to give [Ir-4]@Si500. After isolation of the coated colloidal particles and their redispersion in water, it was noted that the ζpotential of the particles was very close to zero (+4 ± 1 mV), consistent with the incorporation of the monopositively charged iridum(III) complex on the negative silica surface which is expected to lead to charge-neutralization and a decrease in particle mobility in electrophoretic fields. However, the small magnitude of the zeta potential of the particles led to instability and rapid sedimentation of the particles (435 nm using a dichroic and long-pass filter combination and thus encompassing all the light observed in the steady-state emission spectrum, arises from particles rather than from a continuum of free-dye. The particles were generally observed to be monodisperse in appearance and with a uniform seeding density across the region of interest. A steady state emission spectrum taken from the light collected to take the micrograph demonstrated unambiguously that the light emitted arises from the immobilized iridium(III) complex Ir-4 and is not from scattering or reflection by the particles. Conversely, [Ir-4]⊂Si500 gave poor imaging results by epifluorescence microscopy as a 0.05% w/v dispersion in water in a microchannel of depth 100 μm (Figure 3), with particles nonuniformly distributed across the channel and only the largest aggregates, made up from multiply coagulated [Ir4]⊂Si500, are visible. These are much brighter than weaker luminescent [Ir-4]⊂Si500 single nanoparticles and can be easily detected. We therefore concluded that these particles were unsuitable for μ-PIV and did not pursue experiments further.

Ir-4 are critical to its incorporation either on the surface or throughout the interior of the particles. By consideration of the photophysical properties of the cyclometalated parent complex Ir-3 we can draw conclusions on the microenvironment experienced by Ir-4 in both [Ir-4]@ Si500-Z and [Ir-4]⊂Si500. Both Ir-1 and Ir-3 display a UV−vis absorption profile with maxima at 255 nm with a shoulder at 340 nm extending into the near-visible, corresponding to ligand-based singlet−singlet absorption (S0−S1) and direct absorption into the singlet metal-to-ligand charge transfer excited state (1MLCT) respectively. Excitation of Ir-3 in acetonitrile into the absorption band at 355 nm elicited a strong and very broad emission in the 450−850 nm region of the electromagnetic spectrum, with a peak maximum of 575 nm in aerated acetonitrile. The photoluminescence quantum yield of Ir-3 proved to be highly dependent on the extent of aeration in the solution, with measurements made in aerated acetonitrile leading to a quantum yield of 3%, while measurements made in deaerated acetonitrile led to a quantum yield of 16%. Likewise, the lifetime of the emission from Ir-3 recorded in aerated acetonitrile gave a biexponential decay with a 37 ns component (15% weighting) combined with a longer 60 ns component (85%), which increased upon deaeration of the solution to 377 ns (32%) with a longer component of 1.3 μs (68%) also observed. Both the changes in quantum yield and luminescentstate lifetimes reflect the nature of the luminescent state in cyclometalated iridium(III) complexes which arises from the mixing of the ligand-centered singlet (3LC) excited state with the singlet and triplet metal-to-ligand charge transfer states (1MLCT and 3MLCT), the extent of which is dictated by the extent of spin−orbit coupling; as the excited state generally has significant triplet character, oxygen is a potent quencher of the luminescence. Hence, upon deaeration of the solutions, a quenching pathway is removed and both lifetime and quantum yield increase as a result. Taking these observations into account, we can thus make robust conclusions regarding where the luminescent iridium complexes are predominantly located in the nanoparticles. The insensitivity of the observed lifetime of [Ir-4]⊂Si500 to aeration suggests strongly that the iridium(III) dye is located predominantly within the interior 14705

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To test the viability of the particles as flow tracers, we again used epiluminescence microscopy to image the particles in a flow channel within a region of interest close to the channel wall. [Ir-4]@Si500-Z at 0.05% w/v were introduced into the channel, and under flow conditions it was possible to trace their movement using PIV. The PIV analysis was performed on an image sequence of about 990 frames at 20 Hz. The region of interest was of 0.16 × 0.10 mm2 in the proximity of the channel wall and the flow inside the microchannel was along the vertical direction (y) and downward. A Eulerian velocity field was obtained via the ensemble PIV routine over the 990 pairs of images (Figure 4).

Figure 5. Mean velocity profiles obtained by using PIV velocity measurements (○). The theoretical trend relative to the x−y plane at z = 1 μm is superimposed as a solid line ().

channel wall, some discrepancies were observed: the measured velocity is slightly lower than the theoretical values given by the analytical solution. A slight deviation from the straight vertical direction of the flow was in fact observed for particles closer to the channel walls where a positive horizontal component can be observed. The order of this deviation was found to be about 3% of the velocity magnitude in the middle of the channel. We ascribe the deviation effect to repulsion of the particles at the wall; the channel is inherently hydrophobic as it was pretreated with a solution of bovine serum albumin to avoid fouling. However, the hydrophilic phase of the protein is most probably directed toward the bulk solvent and repels the hydrophobic particles due the polarized C−F bonds of the fluorosurfactant. PIV data also showed small local oscillations, possibly due to local poor seeding conditions within the interrogation windows.

Figure 4. Ensemble averaged Eulerian velocity vector field resolved by the PIV routine.

To assess the performance of the particles paying particular emphasis to their following faithfully the theoretical flow profile, we used a model for the analytical solution of the momentum balance for laminar flow inside a channel of rectangular cross section at depth z = 1 μm from the wall. Using the total flow through the channel (Φ), the local flow velocity on the observation plane was modeled according to the following solution proposed by Cornish:51

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CONCLUSION In conclusion, the luminescence studies of silica nanoparticles incorporating iridium(III) complexes by different approaches reveal that the most luminescent particles are prepared by coating the particle with the metal complex and fluorosurfactant. Covalent incorporation of the dye throughout the silica and by surface coating was much more effective giving luminescent particles that simple dye absorption. The fluorosurfactant was needed to stabilize the nanoparticles but also provided an unexpected benefit in that the photophysical properties were substantially improved based on the protection of the fluorosurfactant of the Ir(III) environment. The coated particles were used for μ-PIV measurements in a microchannel and found to follow a theoretical profile for laminar flow in a rectangular channel. We expect these particles not only to be equivalent in terms of their brightness with commercially available particles used for μ-PIV, but also to offer the longterm stability and robustness required for such applications.

where 2h is the depth of the channel in direction of the z-axis (parallel to the flow), 2b is the width of the channel in the direction of the x-axis, and δp/δy the pressure drop along the channel. The y-axis is taken to be the direction of the flow, with the origin of the reference frame is at the center of the channel. The mean profile of the vertical velocity along the x direction was plotted for the PIV results and compared with the expected profile given by the solution to this theoretical model (Figure 5). It was found that the experimentally measured profile followed the theoretical trend identically where the region of uniform velocity starts approximately at 0.1 μm mm away from the channel wall. This is perhaps the most important result for the μ-PIV application in that the particles faithfully follow the flow in the region of uniform velocity. However, close to the

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthetic procedures and characterization for metal complexes and ligands, together with instrumentation details for nanoparticle characterization; luminescence studies; UV− vis, luminescence, NMR, and MS spectra. This material is available free of charge via the Internet at http://pubs.acs.org. 14706

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +44 (0) 121 414 2290. Author Contributions ∥

D.J.L., V.D., and N.J.R. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank EPSRC, the Leverhulme Trust (Z.P.), the School of Chemistry, and University of Birmingham for funding. Some instruments used in this study were supplied through Birmingham Science City: Innovative Uses for Advanced Materials in the Modern World (West Midlands Centre for Advanced Materials Project 2) with support from Advantage West Midlands (AWM) and partial funding from the European Regional Development Fund (ERDF).

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dx.doi.org/10.1021/la403172m | Langmuir 2013, 29, 14701−14708