Technical Note pubs.acs.org/ac
Nanoparticles of Fluorescent Conjugated Polymers: Novel IonSelective Optodes Katarzyna Kłucińska, Emilia Stelmach, Anna Kisiel, Krzysztof Maksymiuk, and Agata Michalska* Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland S Supporting Information *
ABSTRACT: A novel type of ion-selective nano-optode is proposed, in which a conjugated polymer is used as optical transducer and nanoprobe material. Thus, contrary to most of the proposed optodes, the response does not require presence of pH-sensitive dye in the sensor. The conjugated polymer nanosensor material is in partially oxidized formit is bearing positive charges and its emission is quenched. The receptor is an optically silent uncharged ionophore selective for the analyte cation. When a binding event occurs, positive charges are formed in the nanosphere, leading to a decrease in the oxidation state of the polymer, in the absence of redox potential change, resulting in increased emission. This general approach herein proposed results in a simple sensor, benefitting from a novel optical transduction mechanism and high lipophilicity of the polymer matrix that results in linear responses over a broad concentration range of analyte. For the model system studied, the linear dependence of emission intensity on the logarithm of analyte (K+) concentration was obtained for a broad range from 10−5 M to 0.1 M. delocalized π-electrons undergo changes.15−17 Moreover, in a simple procedure (such as, e.g., nanoprecipitation), a dispersion of CPs can be obtained, yielding bright, photostable nanoparticles in solution.18−20 Despite the significant advantages offered by CPs, to the best of our knowledge, they have not been considered as optical transducers for ion-selective optodes. It should be also stressed that the compatibility of CPs (including polythiophenes), with ionophores used as receptors has been proven (see, e.g., refs 21−23). Conveniently, CP fluorescence is observed for the neutral form, whereas the positively charged (partially) oxidized form is optically silent. For example, for poly(3-octylthiophene) (POT), a relatively small change in oxidation state (ca. 100 mV) results in a pronounced change in emission from the polymer film.16,17 In this communication, we explore a novel, alternative approach to formulate nano-optodes resulting in sensors that do not require presence of pH-sensitive dye and show turn-on, competitive performance, including a linear dependence of emission intensity on the logarithm of analyte contents in a broad range. This approach takes advantage of the competition between analyte binding by the ionophore dispersed in the polymer phase, and oxidized/neutral conjugated polymer units, occurring without change of the redox potential. Advantageously, the semiconducting CPs also can serve as a sensor lipophilic matrix. This approach can address different CPs, including those readily available, and various ionophores. As a
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on-selective optodes combine highly selective, optically silent receptors and dyes/transducers yielding the possibility of optical readout of analyte contents changes in the sample. Use of this approach dates back to 1990,1,2 and it is clearly of high interest for different research groups today (e.g., refs 3−7). The applied sensing principle requires incorporation into an inert lipophilic medium (usually plasticized poly(vinyl chloride) or polyacrylate polymer) a probeionophore sensitive to the analyte, and H+ ionophore of different optical properties in protonated and deprotonated formused as the optical transducer.3,7,8 For a typical cation-sensitive system, the operational principle is based on competition between H+ and analyte cation binding in the lipophilic phase. More recent successful research led to micro- and nano-optodes useful for different small sample volume sensing needs [e.g., refs 3, 5, 7−10]. However, the Achilles heel of classical optodes approach is necessity to use H+ selective ionophore as optical transducer and narrow choice of suitable dyes, typically N-octadecanoylNile blue is used (see, e.g., refs 1, 2, 8, 9). Even alternative approaches proposed recently and taking advantage of nonequilibrium sensing mode11 require the presence of a H+sensitive transducer with its drawbacks related to bleaching, etc.12 Typical sigmoidal shape responses are characterized with high sensitivity, for narrow concentration range covering 2−3 orders of magnitude (see, e.g., refs 2 and 7). The system contains minimum three components: a receptor, a transducer, and a polymeric matrix. Clearly, there is significant research interest in overcoming this limitation (see, e.g., refs 13 and 14). Conjugated polymers (CPs), e.g., polythiophenes, are recognized, among others, as important optoelectronic materials. Upon perturbation, the absorption and emission properties of these materials related to the presence of © 2016 American Chemical Society
Received: February 24, 2016 Accepted: May 2, 2016 Published: May 2, 2016 5644
DOI: 10.1021/acs.analchem.6b00737 Anal. Chem. 2016, 88, 5644−5648
Technical Note
Analytical Chemistry
highly lipophilic, a diffusion limitation, with regard to the access of ions to the bulk of nanospheres, can be expected, as we reported previously.13 Thus, ultimately, sensors proposed herein are expected to show linear dependence of emission intensity on the logarithm of analyte concentration within a broad (a few orders of magnitude) concentration range. In the model system studied, K+ ions are preferentially bound by ionophore−valinomycin, affecting the ratio of number of oxidized and neutral polymer units and resulting in emission intensity changes (see Scheme 1).
model system, we have chosen poly(3-octylthiophene) and valinomycin to yield the potassium turn-on sensor.
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THEORETICAL BACKGROUND For CPs, the major change of the redox state is inevitably bound with exchange of ions with the environment (see, e.g., ref 22). Although polythiophenes, including poly(3-octylthiophene) (POT), are relatively stable in a semiconducting, neutral state in the absence of dopants, the introduction of lipophilic anions to the CP allows (partial) spontaneous oxidation by, e.g., dissolved oxygen, similar to that reported earlier.24 Thus, for CP (e.g., POT) containing dopant anions (e.g., introduced cation exchanger) in contact with oxygen present in an aqueous phase, a stable redox state is obtained, with a constant redox potential determined by the ratio of number of the oxidized and neutral polymer units (for simplicity, only CP units that are changing their oxidation state are shown):
Scheme 1. Schematic Representation of Proposed Conjugated Polymers (CPs) Nano-optodes Involving Novel Optical Transduction Mechanism of Ionophore-Based Ion Recognition and Yielding an Optical Signal Resulting from Analyte−Probe Interactions
(CP0 ·R−·N+) ↔ (CP+ ·R−)CP + N+solution + e− CP fluorescent 0
nonfluorescent
(1)
+
where CP and CP represent the neutral and the oxidized form of the polymer (e.g., poly(3-octylthiophene)), respectively; R− is a lipophilic anion and N+ is a cation, CP index refers to the CP phase. The change of the oxidation state of the polymer requires exchange of ions to maintain the electroneutrality of the CP. If the CP0 and CP+ are characterized with different emission properties, e.g., if the neutral form is fluorescent and the oxidized form is not fluorescent, as in the case of polythiophenes, the change of the oxidation state of the polymer can be observed as a change of emission.16,17 If the ion exchange is hindered, the redox state of the polymer cannot be altered, and the change of the emission will not be observed. Accordingly, if the exchange of particular ions is preferred, e.g., by selective complexation within CP by a neutral ligand (in the presence of cation exchanger), the formation of charged complex within the polymer matrix results in disturbance of electroneutrality of the CP phase that can be compensated only by the change of the number of CP0 and the number of CP+. Thus, the binding of the analyte in the nanosphere can be monitored by change of the emission of the system:
The approach proposed herein can be seen as a descendant of pH-sensitive dye applications as optical transducers in the sense that the recognition is based on the selective reaction of the analyte cation with the ionophore, inducing an emission response of the system, to maintain the electroneutrality. However, in the proposed system, the competition is between the analyte cation and the ratio of number of charged and neutral conjugated polymer backbones. Note, H+ ions are not involved in the process. Moreover, nano-optodes proposed herein use CPs as a lipophilic phase (i.e., a sensor matrix), leading to simpler sensor preparation.
(nCP+ ·mCP0 ·R−·L)CP + Mz +solution lower fluorescence
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⇆ ((n − z)CP+ ·(m + z)CP0 · R−· L· M z +)CP higher fluorescence
EXPERIMENTAL SECTION Reagents. Poly(3-octylthiophene-2,5-diyl) (POT), poly(vinyl alcohol) (PVA), cetyltrimethylammonium chloride (CTAC), sodium dodecyl sulfate (NaDS), tetrahydrofuran (THF), valinomycin, sodium tetraphenylborate (NaTFPB), tridodecylmethylammonium chloride (MTDDA-Cl), bis(2ethylhexyl) sebacate (DOS), tris(hydroxymethyl)aminomethane (tris), sodium hexacyanoferrate(II), sodium hexacyanoferrate(III) were obtained from Aldrich (Germany). Other chemicals used included hydrochloric acid, acetic acid and sodium hydroxide; these were of analytical grade and were obtained from POCh (Gliwice, Poland). Doubly distilled and freshly deionized water (resistance = 18.2 MΩ cm, Milli-Q Plus, Millipore, Austria) was used throughout this work.
(2)
where Mz+ is a cation for which ionophore L is selective, n and m are the stoichiometric coefficients, and the rest of the symbols have the same meaning as in eq 1. An increase in the concentration of cations (Mz+) in solution leads to an increase in the number of neutral (f luorescent) polymer units, at the expense of decreasing the number of charged (nonf luorescent) polymer units without a change of the redox potential of the polymer. Since the neutral form of CP is fluorescent, an increase of emission intensity with the increasing concentration of Mz+ ions will be observed. The ionophore-discriminated cations cannot be easily incorporated from solution to the polymer; thus, no change in emission is expected. Moreover, since the polymeric nanoparticles are 5645
DOI: 10.1021/acs.analchem.6b00737 Anal. Chem. 2016, 88, 5644−5648
Technical Note
Analytical Chemistry
spectra obtained for POT nanoparticles is significantly dependent on the surfactant applied to stabilize nanoparticles (Figure S2A). The nanospheres prepared in the presence of cationic surfactant (CTAC) were characterized with higher emission intensities, in comparison to nanoparticles prepared in the presence of anionic surfactant (NaDS) (Figure S2A). In accordance with eq 1, the presence of positive charges on the surface of nanospheres prevents oxidation of the polymer to CP+, which can be observed as an increase in emission, compared to the nanoparticles obtained in the presence of anionic surfactant, where the equilibrium is shifted toward the quenched form (CP+), because of the presence of anions on the surface of nanoparticles. Depending on the charge of surfactant used, different sensitivities for solution redox potential changes were observed (see Figures S2B and S2C in the Supporting Information). For nanospheres prepared in the presence of a cationic surfactant (CTAC), in the absence of plasticizer, the change of the redox potential related to hexacyanoferrate(II)/(III) redox buffer, of oxidized to reduced form concentration ratio changing from 1:10, through 1:1 to 10:1, resulted in a change in the emission spectra recorded. An increase in the concentration of hexacyanoferrate(II) anions in the mixture resulted in increased emission, which can be attributed to lower redox potential of the solution and preferred presence of the CP0 form of the polymer. Accordingly, the increase of hexacyanoferrate(III) anions contents resulted in a decrease in emission, because of partial oxidation of the polymer and preferred formation of CP+. It is expected that, in this case, the charge balance is maintained by solution anions that can accumulate close to the nanosphere, because of the cation-exchanging properties of the anions of the applied surfactant. On the other hand, nanoparticles prepared in the presence of an anionic surfactant (NaDS), in the absence of a plasticizer, were significantly less affected by the change of the solution redox potential (see Figure S2C). Virtually the same spectra were obtained, regardless of changes in the redox potential. It should be stressed that the nanospheres obtained in the presence of a small amount of plasticizer, as reported previously for potentiometric applications of POT films,11,21 regardless of the surfactant used, were practically insensitive for solution redox potential change (see Figures S2B, S2C, and S2D in the Supporting Information). Despite the fact that the ratio of concentration of oxidized and reduced form of buffer constituents corresponds to a solution redox potential change of 150 mV, and regardless of the ion-exchange properties of applied surfactant, within the range of experimental error, the same emission spectra were obtained. It seems probable that the beneficial insensitivity toward redox reactants results from significant lipophilicity of the nanoprobe material, which further increases with the addition of plasticizer. Thus, further experiments were performed for plasticizer containing POT nanospheres. The emission intensity obtained for POT nanospheres, and nanospheres loaded with cation exchanger, or cation exchanger and ionophore in the absence of K+ ions (Figure S3 in the Supporting Information), differ significantly. As it can be seen in Figure S3, POT nanospheres in the absence of ionophore and cation exchanger, are characterized with strong emissions, with two POT characteristic maxima located at ∼655 and 720 nm, pointing to the semiconducting (mostly neutral) form of the CP in nanoparticles. If the anion exchanger (MTDDA-Cl) was introduced to the nanoparticles, the emission observed was
The buffer used was 0.1 M tris (adjusted with HCl) to pH 7.8. Apparatus. Fluorimetric experiments were performed using a spectrofluorimeter (Varian, Cary Eclipse). After excitation at a wavelength of 550 nm, the emission intensity was recorded within the range of 600−800 nm. Unless otherwise stated, the slits used were 10 nm both for excitation and emission, while the detector voltage was maintained at 750 V. Ultraviolet−visible light (UV-vis) experiments were performed using a PerkinElmer Model LAMBDA 650 UV/vis spectrophotometer. To obtain transmission electron microscopy (TEM) images of prepared nanospheres, a Zeiss LIBRA 120 (HT = 120 kV, LaB6 cathode) TEM apparatus was used. The obtained nanoparticles were characterized using Zetasizer Nano ZS Malvern (scattering angle = 173°). Synthesis of Nanospheres. Five milliliters (5 mL) of solution of aqueous 1% (w/w) PVA was placed on a stirring plate (600 rpm) and 500 μL of POT solution in THF (10 mg/ mL) was added. The mixture was left on a stirring plate in an open vial. One milliliter (1 mL) of obtained POT nanoparticles suspension was placed on a stirring plate (600 rpm) and 200 μL of THF solution typically containing 8 mg of valinomycin, 1 mg of NaTFPB, and 15 mg of DOS in 1 mL was added; unless otherwise stated, these nanospheres were used. For a control experiment on the effect of compounds introduced to POT on emission spectra, nanospheres prepared using either of the above components have been used. Alternatively, 500 μL of POT solution in THF (10 mg/mL) was introduced to either 0.1 M CTAC or 0.1 M NaDS solution, or these nanospheres were additionally modified by introducing DOS, added as described above. Unless otherwise stated, PVA solution was used. After preparation, the nanospheres suspension was left on a stirring plate in an open vial for 30 min. Samples for TEM Imaging. The nanosphere suspension was diluted 10 times with deionized water and dropcast on formvare (grit mesh 400), and the nanoprobes were left to dry at room temperature. Samples for DLS Measurements. One hundred microliters (100 μL) of the nanosphere suspension was mixed with 3 mL of tris buffer solution. Optical Measurements. For both absorption and emission measurements, 30 μL of the nanosphere suspension was mixed with 3 mL of buffer optionally containing analyte. Redox buffers were prepared by spiking the buffer used with sodium hexacyanoferrate(II) and sodium hexacyanoferrate(III) of concentrations equal to 0.01 M, mixed in the following volume ratios: 1:10, 1:1, and 10:1 or 1:20, 1:1, and 20:1.
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RESULTS AND DISCUSSION Using the nanoprecipitation method, CP nanostructures were formed from poly(3-octylthiophene) solution. The diameter of resulting structures was close to 250 nm both under the conditions of TEM characterization (Figure S1 in the Supporting Information), and using the DLS approach (Figure S1). The nanoparticles obtained were stable in water (for more than 2 weeks). On spectra obtained for POT prepared in the presence of PVA, cationic or anionic surfactant, peaks characteristic for neutral semiconducting POT nanoparticles,17 with maxima at ∼655 and 720 nm were observed (see Figure S2A in the Supporting Information). However, the intensity of emission 5646
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Figure 1. Emission intensity at 655 nm versus the logarithm of concentration dependence; results were obtained in 0.1 M tris buffer pH 7.8 of plasticized POT nanospheres loaded with NaTFPB and valinomycin for different concentrations of K+ cations (from 10−6 M to 0.1 M) and emission spectra in the presence and the absence of K+ cations in solution (red line), excitation wavelength = 550 nm, slits 10/10 nm, detection voltage = 800 V.
to those recorded in the absence of redox buffer in solution (within the experimental error). These results are in accordance with the above-reported insensitivity of POT-plasticized nanospheres for changes of redox potential. It is also highly promising for the practical applications of herein proposed nano-optodes system. It should be stressed that, by keeping the amount of introduced dopants (ion exchanger) low, only partial oxidation of POT was enabled, and, thus, the lipophilicity of the system was not significantly compromised. The mechanism of optical responses proposed herein is dependent on the affinity of applied ionophore for the analyte ions, which, for the sake of simplicity, can be approximated by the potentiometric selectivity of the ionophore. It can be expected that emission for different interfering ions present in the solution (in the absence of primary ions) will be dependent on the selectivity of ionophore. In fact, for model interfering ions tested in the absence of K+ ions in the sample (NH4+, Na+, Ca2+, Mg2+), within the concentration range from 10−5 M to 10−2 M for monovalent ions and from 10−5 M to 10−3 M for divalent ions, no change of the intensity of emission was observed (see Figure S7 in the Supporting Information). It should be stressed that the spectra of POT nanoparticles containing an ion exchanger were independent of changes of sample pH (see Figure S8 in the Supporting Information). Some increase in the emissions was observed only for higher concentrations of divalent interfering ions (>10−3 M) solutions; however, it should be stressed that intensities recorded even for 0.1 M solutions of Ca2+ and Mg2+ were significantly lower compared to those observed in solutions containing K+ cations (see Figure S7). Interestingly, the intensities (at maximum) obtained for Na+ or NH4+ were 7.6 and 4.0 times smaller, respectively, compared to the signals obtained in K+ solutions. This result is in accordance with the selectivity coefficient of potassium-selective electrodes, for highly lipophilic systems, e.g., polyacrylate membranes; the logarithm of the selectivity coefficients obtained for Na+ ions is ∼2 units lower, compared to the value obtained for NH4+ ions.26 On the other hand, emission intensities obtained for Ca2+ or Mg2+ ions were comparable to each other and (at maximum) were ∼4 times smaller, compared to that obtained for K+ ions of equivalent concentration. However, in this case, direct comparison is not possible, because the potentiometric selectivity coefficient for interfering ions of different valency from primary ions takes
comparable with that of unmodified POT nanoparticles (results not shown). On the other hand, the introduction of a cation exchanger has led to pronounced quenching of emission of the system, typical for partially oxidized polymer, in accordance with eq 1. A similar effect was observed for the system containing both ion-exchanger and ionophore. It should be stressed that the absorption spectra recorded for suspension of POT nanospheres or POT nanoparticles loaded with ionophore and ion-exchanger, under the same conditions, were unaffected within the range of experimental error (Figure S4), clearly indicating the high sensitivity of the system in the emission mode. Figure 1 shows the response of the herein described nanooptodes containing valinomycin for the change in the concentration of K+ ions in solution. As can be seen, the increase in the concentration of K+ ions in solution resulted in an increase in the emission of nano-optodes, as expected according to eq 2. The linear dependence of emission on the logarithm of analyte concentration in the sample was observed for a broad concentration range, from 10−5 M to 10−1 M (R2 = 0.990), which is significantly broader, compared to ion-optodes working in exhaustive mode.25 The broad linear dependence of emission on the logarithm of analyte concentration is not common for optical sensors; however, it was reported for probes with significant limitation in transport of the analyte in the probe.5,10,13 It seems probable that, also for POT nanooptodes, the transport of analyte ions to the bulk nanospheres is hindered because of the lipophilic nature of poly(3octylthiophene). As expected for prevailing transport limitation in longer time scales of sample−probe contact (ca. 10 h), the response pattern recorded was more similar to a sigmoidal-type response (see Figure S5 in the Supporting Information). Since the transduction system proposed herein is based on the change of ratio and number of neutral and charged CP units, the effect of the presence of redox buffer in the sample was tested, using buffers of oxidized to reduced form (with concentration ratios changing from 1:20 to 20:1). In accordance with the above-discussed results, and as it can be seen in Figure S6 in the Supporting Information, the presence of redox buffers of different redox potential did not result in a change in the emission intensities recorded, despite the fact that the redox potential of the solutions differed for ∼150 mV (see Figure S6). Moreover, the emission values recorded were equal 5647
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(5) Kisiel, A.; Kłucińska, K.; Głębicka, Z.; Gniadek, M.; Maksymiuk, K.; Michalska, A. Analyst 2014, 139, 2515−2524. (6) Dubach, J. M.; Harjes, D. I.; Clark, H. A. J. Am. Chem. Soc. 2007, 129, 8418−8419. (7) Ruedas-Rama, M. J.; Walters, J. D.; Orte, A.; Hall, E. A. H. Anal. Chim. Acta 2012, 751, 1−23. (8) Ruedas-Rama, M. J.; Hall, E. A. H. Analyst 2006, 131, 1282− 1291. (9) Clark, H. A.; Hoyer, M.; Philbert, M. A.; Kopelman, R. Anal. Chem. 1999, 71, 4831−4836. (10) Kisiel, A.; Kłucińska, K.; Gniadek, M.; Maksymiuk, K.; Michalska, A. Talanta 2015, 144, 398−403. (11) Xie, X.; Zhai, J.; Bakker, E. Anal. Chem. 2014, 86, 2853−2856. (12) Langmaier, J.; Lindner, E. Anal. Chim. Acta 2005, 543, 156−166. (13) Woźnica, E.; Maksymiuk, K.; Michalska, A. Anal. Chem. 2014, 86, 411−418. (14) Xie, X.; Gutiérrez, A.; Trofimov, V.; Szilagyi, I.; Soldati, T.; Bakker, E. Anal. Chem. 2015, 87, 9954−9959. (15) Wang, W.; Lin, J.; Cai, Ch.; Lin, S. Eur. Polym. J. 2015, 65, 112− 131. (16) Danno, T.; Kobayashi, K.; Tanioka, A. J. Appl. Polym. Sci. 2006, 100, 3111−3115. (17) Palacios, R. E.; Barbara, P. F. J. Fluoresc. 2007, 17, 749−757. (18) Feng, L.; Zhu, Ch.; Yuan, H.; Liu, L.; Lv, F.; Wang, S. Chem. Soc. Rev. 2013, 42, 6620−6633. (19) Wu, Ch.; Chiu, D. T. Angew. Chem., Int. Ed. 2013, 52, 3086− 3109. (20) Pecher, J.; Mecking, S. Chem. Rev. 2010, 110, 6260−6279. (21) Bobacka, J.; Lindfors, T.; McCarrick, M.; Ivaska, A.; Lewenstam, A. Anal. Chem. 1995, 67, 3819−3823. (22) Bobacka, J. Electroanalysis 2006, 18, 7−18. (23) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537−2574. (24) Michalska, A.; Skompska, M.; Mieczkowski, J.; Zagorska, M.; Maksymiuk, K. Electroanalysis 2006, 18, 763−771. (25) Xie, X.; Zhai, J.; Crespo, G. A.; Bakker, E. Anal. Chem. 2014, 86, 8770−8775. (26) Michalska, A. J.; Appaih-Kusi, Ch.; Heng, L. Y.; Walkiewicz, S.; Hall, E. A. H. Anal. Chem. 2004, 76, 2031−2039.
into account the charge difference. For the nano-optodes containing valinomycin ionophore and anion exchanger (MTDDA-Cl), i.e., nanospheres containing highly lipophilic cations that cannot be exchanged with solution, virtually no emission change for the change in the concentration of K+ ions in solution was observed (results not shown).
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CONCLUSIONS Novel nano-optodes that have been proposed herein, using as sensor material and optical transducer polythiophenes offer significant improvements compared to existing systems. The optical sensors obtained show linear dependences of emission on the logarithm of analyte concentration within a broad range (5 orders of magnitude) in turn-on mode. In contrast to the classical optodes system, the presence of H+-sensitive ionophores in the nanosensor is not required. The change of observed emission is driven by the change of the ratio of the number of oxidized and neutral polymer units in the nanoparticle. The obtained analytical benefits prove that conjugated polymerswhich, here, have been studied based on the example of poly(3-octylthiophene), which is a model representative of large family of compoundsare attractive transducers for optode nanosensors. Similar behavior, within the range of experimental error, was also observed for poly(3dodecylthiophene)-based nanoparticle sensors. It should be also stressed that these versatile sensors are easy to prepare in course of nanoprecipitation and the nanospheres obtained are stable over time.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00737. Discussion of the testing process for ion-selective nanooptodes; description of the morphology and size of the poly(3-octylthiophene) (POT) nanoparticles produced; emission spectra of nanospheres used in the study; absorption spectra of the nanoparticles (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +48 22 5526 331. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Authors are grateful to Ph.D. candidate M. Gniadek for TEM imaging and M.Sc. candidate Z. Głeb̨ icka for DLS measurements. Financial support from National Science Centre (NCN, Poland) (Project No.2015/15/B/ST4/04909), in the years 2015−2018, is gratefully acknowledged.
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REFERENCES
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