Doped Silica Nanoparticles for Sensing - American Chemical Society

Dec 15, 2014 - Interactions on Substrate-Induced Luminescent Response. Alsu R. Mukhametshina ... Kazan Federal University, Kremlyovskaya str. 18, 4200...
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Tb(III)-Doped Silica Nanoparticles for Sensing: Effect of Interfacial Interactions on Substrate-Induced Luminescent Response Alsu R. Mukhametshina,† Asiya R. Mustafina,*,‡ Nikolay A. Davydov,‡ Svetlana V. Fedorenko,‡ Irek R. Nizameev,§ Marsil K. Kadirov,‡ Valery V. Gorbatchuk,† and Alexander I. Konovalov† †

Kazan Federal University, Kremlyovskaya str. 18, 420008, Kazan, Russian Federation A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Arbuzov str. 8, 420088, Kazan, Russian Federation § Kazan National Research Technological University, K. Marx str. 68, 420015, Kazan, Russian Federation ‡

S Supporting Information *

ABSTRACT: The present work introduces the easy modification of the water-in-oil microemulsion procedure aimed at the doping of the Tb(III) complexes within core or shell zones of the silica nanoparticles (SNs), which are designated as “core-shell”, “shell”, and “core”. The dye molecules, chelating ligands, and copper ions were applied as the quenchers of Tb(III)-centered luminescence through dynamic or/and static mechanisms. The binding of the quenchers at the silica/water interface results in the quenching of the Tb(III) complexes within SNs, which, in turn, is greatly dependent on the synthetic procedure. The luminescence of “core” SNs remains unchanged under the binding of the quenchers at the silica/water interface. The quenching through dynamic mechanism is more significant for “core−shell” and “shell” than for “core” SNs. Thus, both “core−shell” and “shell” SNs have enough percentage of the Tb(III) complexes located close to the interface for efficient quenching through the energy transfer. The quenching through the ion or ligand exchange is most efficient for “core−shell” SNs due to the greatest percentage of the Tb(III) complexes at the silica/water interface, which correlates with the used synthetic procedure. The highlighted regularities introduce the applicability of “core− shell” SNs used as silica beads for phosphatidylcholine bilayers in sensing their permeability toward the quenching ions.

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encapsulated into a silica matrix.25−27 Nevertheless, permeation of small cations and anions into a silica matrix can induce the inner sphere variations of the silica-coated lanthanide complexes,27,28 which affects the lanthanide-centered luminescence. The published data are not enough to recognize the contribution of the inner sphere variations through the ion or ligand exchange to the substrate-induced response of lanthanide complexes within silica nanoparticles. Although both the shielding effect of the silica coating and its permeability are well-known, their effect on the substrate-induced response of the lanthanide complexes doped into silica nanoparticles (SNs) is not well understood. The present work is aimed at the estimation of the factors affecting the substrate-induced response of silica-coated Tb(III) complexes. The efficiency of static and dynamic quenching mechanisms was correlated with the distribution of Tb(III) complexes between various zones of the silica matrix and at the silica/water interface. The distribution can be varied by the modification of the synthetic procedure. Deposition of a phospholipid bilayer onto silica nanoparticles is the well

INTRODUCTION Lanthanide-based labels and sensors are of particular importance in bioanalysis.1−5 Lanthanide-centered luminescence is a key reason for lanthanide application in sensing and imaging. An energy transfer, in particular a ligand-to-metal energy transfer, is a well-known reason for a lanthanidecentered luminescence response to a complex formation of lanthanide ions.1 In turn a ligand environment of lanthanide ions can be easily changed by external stimuli, such as pH or substrate variation.3−12 Stimuli-responsive lanthanide-centered luminescence has wide application in the development of sensing procedures.1−12 Moreover, the impact of the Forster mechanism in quenching of the lanthanide-centered luminescence opens a way to quench the luminescence of lanthanide complexes by their outer sphere binding or collision with molecules or ions with specific spectral properties.13−17 Quenching of molecular complexes in solutions through an outer sphere energy transfer mechanism is often accompanied by inner sphere variations, including a ligand exchange resulting from chelating anions of buffer systems. The encapsulation of lanthanide complexes into a silica matrix is the well-known route to limit undesirable inner sphere variations.2,18−24 Our previous works highlight the increased impact of dynamic mechanism in the quenching of lanthanide complexes © XXXX American Chemical Society

Received: August 1, 2014 Revised: December 2, 2014

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the aqueous dispersion of SNs (4 g·L−1) with both vesicle and Tris buffer solutions, followed by stirring of the mixture within 30 min. The samples of SN aqueous dispersions for DLS and luminescence measurements were prepared in Tris buffer solutions (10 mM, pH 7.4). The initial buffer solutions in turn were prepared from bidistilled water with further filtering through nylon membrane filter (0.45 μm, Millipore). All samples were ultrasonicated for 1 h at 25 °C before measurements. Methods. Transmission electron microscopy (TEM) images were obtained with a Hitachi HT7700, Japan. The images were acquired at an accelerating voltage of 100 kV. Samples were ultrasonicated in absolute ethanol for 10 min and then dispersed on 200 mesh copper grids with continuous formvar support films. The DLS measurements were performed by means of a Malvern Mastersize 2000 particle analyzer. A He−Ne laser operating at 633 nm wavelength and emitting vertically polarized light was used as a light source. The measured autocorrelation functions were analyzed by Malvern DTS software and the second-order cumulant expansion methods. The effective hydrodynamic diameter (D) was calculated by the Einstein−Stokes relation from the first cumulant. The average error in these experiments was approximately 4%. A zetasizer potential “Nano-ZS” (MALVERN) device was used for zeta potential measurements. The steady-state emission spectra were recorded on a spectrofluorometer Fluorolog-3-221-NIR (JobinYvon) under excitation at 330 nm. Luminescence decay was measured using a Fluorolog-3-221 spectrofluorometer with a SPEX FL-1042 phosphorimeter accessory and a xenon flash lamp as the photon source with the following parameters: time per flash, 50.00 ms; flash count, 200 ms; initial delay, 0.05 ms; and sample window, 2 ms. Excitation of samples was performed at 330 nm, and emission was detected at 546 nm with 2 nm slit width for both excitation and emission. All measurements were made at least three times at 25 °C. The experimental points are the mean of at least three measurements. The standard deviation value for each experimental point was calculated on the basis of at least three measurements.

documented route of their surface decoration due to its fundamental and practical importance. Silica nanoparticles coated by phospholipid bilayers29−31 exemplify hard−soft colloids, which have gained great attention in recent decades due to their applicability in mimicking processes taking part in cell membranes32 or in improving the blood compatibility of nanoparticles.33,34 Thus, the effect of phosphatidylcholine (PC) bilayers on the substrate-induced response of the Tb(III) complexes doped into SNs is of particular interest from the viewpoint of their application in imaging or sensing. In particular, the use of Tb(III)-doped SNs as silica beads for PC bilayers may be a possible route to reveal their permeability toward ions and molecules, which can quench Tb(III)-centered luminescence.

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EXPERIMENTAL SECTION

Reagents and Materials. Tetraethyl orthosilicate (TEOS, 98%), ammonium hydroxide (28−30%), n-heptanol (98%), cyclohexane (99%), adenosine (99%), adenosine-5′-monophosphate (AMP, 99%), adenosine-5′-triphosphate disodium salt hydrate (ATP, 98%), guanosine-5′-monophosphate disodium salt hydrate (GMP, 97%), guanosine-5′-triphosphate disodium salt hydrate (GTP, 90%), Na2EDTA (ethylenediaminetetraacetic acid disodium salt dihydrate), and CuSO4·5H2O were purchased from Acros and used without further purification. Terbium(III) nitrate hexahydrate (99.9%) from Alfa Aesar, tris(hydroxymethyl)-aminomethane (Tris) from Scharlau, Triton X-100, merocyanine 540, and guanosine (98%) from SigmaAldrich, and L-α-phosphatidylcholine (PC, ≥99%) from Sigma-Aldrich were used as received. Synthesis of Silica Nanoparticles Doped with Tb(III). Synthesis of “core−shell” Tb(III)-doped silica nanoparticles was performed in accordance with the previously reported modified reverse microemulsion procedure.19 The reverse microemulsion procedure was modified for the synthesis of “shell” silica nanoparticles as follows. The mixture of Triton X-100 (8.63 g), n-heptanol (8.1 mL), cyclohexane (33.75 mL), and water (2.16 mL) was prepared and stirred for 15 min. Then aqueous ammonia (0.27 mL, 28-30%) was added to the mixture, followed by addition of TEOS (0.23 mL) in 15 min. After stirring for the next 24 h, TEOS (0.23 mL) was added together with an aqueous solution of Tb-TCAS complex (2.2 mL, CTb‑TCAS = 7.8 mM). The microemulsion was stirred for 30 min with further addition of TEOS (0.23 mL). Afterward, the reaction mixture was continuously stirred for 24 h. For the synthesis of “core” silica nanoparticles, the modification of the reverse microemulsion procedure was as follows. The mixture of Triton X-100 (8.63 g), n-heptanol (8.1 mL), cyclohexane (33.75 mL), and aqueous solution of Tb−TCAS complex (2.16 mL, CTb‑TCAS = 7.8 mM) was stirred for 15 min. Then aqueous ammonia (0.27 mL, 2830%) was added, followed by the addition of TEOS (0.23 mL) after 15 min. After further 24 h of stirring, more TEOS (0.23 mL) and water (2.2 mL) were added. The mixture was stirred for 30 min, and TEOS (0.23 mL) was added once again. Afterward, the reaction was continued under stirring for another 24 h. The silica nanoparticles were precipitated from the microemulsion by adding acetone, centrifuging, and washing by ethanol−acetone mixture (1:1), ethanol (2 times), and water (several times). We used ultrasonication while washing the silica nanoparticles to remove surfactant and physically absorbed Tb−TCAS complex from the surfaces of silica nanoparticles. Preparation of Vesicles. The vesicle preparation has been performed by the dissolution of PC (0.4 g·L−1) in bidistilled water with further stirring within 30 min and ultrasonication for 1 h at 25 °C.35,36 It is worth noting that PC exists in liquid crystalline phase at these temperature conditions. The samples for dynamic light scattering (DLS) measurements have been prepared by the dilution of the initial vesicle solution by Tris buffer solution (10 mM). The interaction of phospholipid vesicles with SNs was induced by mixing the aliquots of

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RESULTS AND DISCUSSION Tb(III)-Centered Luminescence of “Core”, “Core− Shell” and “Shell” SNs. The doping of Tb(III) complex with p-sulfonatothiacalix[4]arene (TCAS)37 into SNs was made through the water-in-oil microemulsion procedure,19 where the addition of both the Tb(III) complex and TEOS in the molar ratio about 1:100 (Tb:TEOS) occurs in one step (Scheme 1a). According to our previous results19 this doping procedure provides more or less homogeneous distribution of luminophores within SNs, which will be further designated as the Scheme 1. Schematically Represented Procedures Applied in the Synthesis of “Core−Shell” (a), “Shell” (b), and “Core” (c) SNs

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Figure 1. TEM images and schematic representation of “core−shell”, “shell” and “core” SNs.

“core−shell”. Thus, prepared “core−shell” SNs exhibit substrate-induced response ability, which indicates the availability of the Tb(III) complexes at the silica/water interface to both static and dynamic quenching by dye molecules and copper ions.27 This doping procedure was modified with the aim to concentrate the Tb(III) complexes in the core or in the shell zones of the SNs and to decrease the concentration of the Tb(III) complexes at the silica/water interface (Scheme 1b). So, the synthesis of the SNs with Tb(III) complexes mostly concentrated in the shell was performed by the preliminary synthesis of the “empty” silica cores with their further coating by an additional silica layer in the presence of Tb(III) complexes with the Tb:TEOS ratio being about 2:100. Next more TEOS was added to make the final Tb:TEOS ratio equal to 1:100. The predominant concentration of the Tb(III) complexes in the core was reached by synthesis of the Tb(III)-doped silica cores at 2:100 ratio (Tb:TEOS) with two steps of further coating by the silica layer through TEOS hydrolysis without Tb(III) (Scheme 1c). This procedure results in the “core” SNs with the same total Tb:TEOS ratio as in the “shell” ones. TEM reveals 35 ± 7, 54 ± 8, and 45 ± 9 nm sized nanoparticles for the “core−shell”, “shell”, and “core” SNs correspondingly (Figure 1). The sizes of the “core” and “shell” SNs are larger than those of the “core−shell” SNs. The larger sizes of “core” and “shell” SNs should be explained by the greater TEOS concentration (56 mM) used in their synthetic procedures as compared with this value for “core−shell” SNs (31 mM). The size difference between “core” and “shell” SNs is within the observed experimental variation. DLS measurements for SNs of three types at a concentration of 0.028 g·L−1 in aqueous buffer solution (Tris, pH 7.4) revealed their size being in the range of 120−150 nm and polydispersity indexes within 0.14−0.16 (Table S1 in the Supporting Information (SI)). These sizes are greater than those obtained from the TEM measurements, which can be

explained by the aggregation of SNs in the aqueous buffer solutions. Figure 2 represents the spectra of the steady state luminescence of the “core”, “shell” and “core−shell” SNs in

Figure 2. Emission spectra of “core−shell” (1), “shell” (2), and “core” (3) SNs (0.028 g·L−1) in aqueous solution at pH 7.4, excited by 330 nm.

aqueous buffer solutions excited at 330 nm. The observed spectrum patterns correspond to 5D4 → 7F6 (494 nm), 5D4 → 7 F5 (545 nm), 5D4 → 7F4 (587 nm), 5D4 → 7F3 (625 nm) transitions, which are peculiar for Tb(III)-centered luminescence, sensitized by the TCAS ligand.1,35 The steady state luminescence of “core” and “shell” colloids has similar intensities, which are two-fold less than that of “core-shell” colloids in the same concentration conditions (Figure 2). This fact correlates with the different amounts of luminescent complexes inside the SNs resulting from the corresponding synthetic conditions. In particular the final Tb(III):TEOS ratio in “core−shell” SNs (2:100) is twice as much as in both “core” and “shell” ones (1:100). The time-resolved luminescence C

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Scheme 2. Schematic Presentation of the Processes Resulting in the Quenching of the Tb(III)-Centered Luminescence

Figure 3. I/I0 (1,3,5) and τ/τ0 (2,4,6) values of “core−shell” (1,2), “shell” (3,4), and “core” (5,6) SNs (0.028 g·L−1) at various concentrations of MC540 (a) and Na2EDTA (b) at pH 7.4. The SD values are designated by error bars.

are exemplified for merocyanine 540 (MC540). The quenching by MC540 arises from the adsorption of the latter onto a silica surface and/or the collision of MC540 with SNs. The contributions of these processes to the quenching are expected to be the same for the studied SNs at the same concentration of MC540. The intensity of the main band at 546 nm arisen from the 5D4−7F5 transition was used for a quantitative evaluation of luminescence response on the external stimuli. Thus, the quenching measurements are represented in the form of I/I0 and τ/τ0, where I and I0 are the intensities, and τ and τ0 are the lifetime values in the presence of the quenchers and in the aqueous buffer solutions correspondingly (Figure 3). The obtained data (Figure 3) indicate the deviation between the steady state and time-resolved quenching measurements, which results from the contribution of the inner filter effect arisen from an electronic absorption of MC540 in aqueous solutions. Nevertheless, this contribution is the same for the studied SNs, while both the steady state and time-resolved luminescence data reveal the difference between the types of SNs (Figure 3). This difference may be explained by the different contribution of dynamic quenching mechanism, which is insignificant for “core” SNs, but high enough for “shell” and “core−shell” SNs. This tendency indicates that the distance between the complexes concentrated within the core and the dye molecules located at the silica/water interface is long enough to prevent an efficient Tb(III)-to-dye energy transfer, which confirms the location of the Tb(III) complexes in the core. The profiles of observed τ/τ0 dependences on the concentration of MC540 for the aqueous colloids of “core−

measurements gave nearly the same lifetime values of the excited state (τ) of 0.95−1.0 ms for all three types of SNs. According to results of our previous work, “core−shell” SNs exhibit rather poor leakage of the Tb(III) complex from the SNs under continuous stirring of their aqueous dispersions for 3 days.19 The leakage of the Tb(III) complex from the “core” and “shell” SNs is also insignificant (Figure S1 in the SI). Scheme 2 represents possible quenching mechanisms for the silica coated Tb(III) complexes. The substitution of TCAS by chelating anions results in quenching of the Tb(III) complexes through the static mechanism (Scheme 2a), if chelating anions have no antennae effect on the Tb(III)-centered luminescence. Thus, ethylenediaminetetraacetic acid disodium salt (Na2EDTA) and nucleotides were chosen as chelating anions. The dye molecules with poor complex ability toward lanthanide ions and specific spectral properties quench Tb(III)-centered luminescence through the dynamic mechanism (Scheme 2b).25,26 The use of Cu(II) ions provides quenching through both dynamic and static quenching mechanisms. In particular, the adsorption of Cu(II) ions at the silica/water interface is the reason for quenching through the static mechanism due to the ion exchange (Scheme 2c), although the contribution of the dynamic mechanism is also detectable.27 The Quenching of “Core”, “Core−Shell” and “Shell” SNs by Dye Molecules. The energy transfer from Tb(III) complexes to the dyes merocyanine 540 and phenol blue was previously reported as the predominant dynamic mechanism of quenching for the silica-coated Tb(III) complexes.25,26,38 In the present work, regularities of the dynamic quenching mechanism D

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Figure 4. I/I0 (1,3,5) and τ/τ0 (2,4,6) values of “core-shell” (1,2), “shell” (3,4) and “core” (5,6) SNs (0.028 g·L−1) at various concentrations of substrates: AMP (a), ATP (b), GMP (c), GTP (d), adenosine (e), and guanosine (f) at pH 7.4. The SD values are designated by error bars.

shell” and “shell” SNs are rather close to each other, which indicates that both types of the SNs have a similar percentage of Tb(III) complexes located at a definite distance (no more than 7 nm17,39) from the quenchers at the interface. The Quenching of “Core”, “Core−Shell” and “shell” SNs by Chelating Anions. As it has been mentioned above, the chelating anions can substitute TCAS from the inner sphere of Tb(III), which should be evident from the quenching of Tb(III)-centered luminescence. To study this effect, both I/I0 and τ/τ0 values were measured under addition of various amounts of Na2EDTA, adenosine-5′-monophosphate (AMP), adenosine-5′-triphosphate disodium salt (ATP), guanosine-5′monophosphate disodium salt (GMP), and guanosine-5′triphosphate disodium salt (GTP) to the aqueous solution of Tb-TCAS (0.01 mM) at pH 7.4. The results reveal significant quenching of the Tb(III)-centered luminescence through the static mechanism at 0.01 mM of ATP, GTP, and EDTA2− (Figure S2 in SI). The effect of AMP, GMP, and nucleosides (exemplified by guanosine) on the Tb(III)-centered luminescence (Figure S2 in SI) is insignificant even at their high excess (0.1 mM) over Tb-TCAS concentration. This tendency is in a good agreement with the ability of the nucleotides and nucleosides to form complexes with lanthanide ions.5−8,40

The fluorescent response of the Tb(III) complexes, doped into “core”, “shell” and “core−shell” SNs on EDTA2− also reveals the difference between the types of the SNs (Figure 3b). Both “core” and “shell” SNs give an insignificant response on EDTA2−, while “core−shell” SNs are significantly quenched at the same concentrations of EDTA2− through the static mechanism without any contribution of dynamic mechanism (Figure 3b). The similar tendency is observed for the “free” Tb(III) complexes in the buffer solutions (Figures 3b and S2 in SI), which points to the ligand exchange (Scheme 2a) as the cause of the quenching of “core−shell” SNs in solutions of EDTA2−. The measurements of the SNs quenching in solutions of the nucleotides and nucleosides also indicates its significant dependence on the doping procedure. The quenching is the most for “core−shell”, is smaller for “shell”, and comes to nothing for “core” SNs (Figure 4). The time-resolved quenching measurements reveal the insignificant contribution of the dynamic mechanism to the quenching, which confirms the ligand exchange as the main cause of the observed quenching. The efficiency of the ligand exchange (Scheme 2a) should depend on the concentrations of both the ligands and the Tb(III) complexes at the silica/water interface. The low E

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Langmuir fluorescent response of “core” SNs to the studied substrates agrees well with the much lower percentage of the Tb(III) complexes at the interface due to their deeper encapsulation as compared with “core−shell” and “shell” ones. The enhanced decrease of the I/I0 values of “core−shell” versus “shell” SNs can be explained by the greater percentage of the Tb(III) complexes at the silica/water interface due to the difference in doping procedures for these SNs (Scheme 1). It is worth noting that the quenching regularities for the Tb(III) complexes within the SNs differ from those for the complexes in solutions (Figure 4 and Figure S2 in the SI). This difference is highlighted for “core-shell” SNs having a greater luminescent response. In particular, the quenching of the luminescence in the colloids of “core−shell” SNs by AMP is more than by ATP (Figure 4), which disagrees with their ability to complex with lanthanide ions.5−8 This fact can be recognized taking into account the impact of the interfacial binding of the chelating anions in the quenching of the silica-coated Tb(III) complexes. The electrokinetic potential values for the SNs are about −35 mV (Table S1 in the SI) in Tris buffer solutions. Thus, the negatively charged surface of the SNs is the reason for the poor binding with triply charged chelating anions (ATP and GTP) due to the electrostatic repulsion. The greater quenching of the luminescence of the SNs by GMP and GTP than by AMP and ATP seems unexpected, since their complex formation with lanthanide ions is rather similar, which is confirmed by the quenching measurements in the solutions (Figure S2 in the SI). Moreover, the quenching effect of both adenosine and guanosine on the I/I0 and τ/τ0 values is much greater in the colloids of the “core−shell” SNs than in the solutions of the “free” Tb(III) complexes (Figure 4e,f and Figure S2 in the SI). The presence of the acidic N−H group in the structure of GTP, GMP, and guanosine, which may enhance the binding of guanosine, GMP, and GTP with the silica surface through the hydrogen bonding between N−H and Si−O groups, can be the reason for their peculiar behavior. The literature results show the possibility of the hydrogen bonding of guanosine with anions via its NH and NH2 groups.41 So, the fluorescent response of the Tb(III)-centered luminescence on the chelating anions depends on the SNs type, which indicates the effect of the doping procedure on the percentage of the Tb(III) complexes at the silica/water interface. Additionally, the adsorption of the quenching anions at the silica/water interface is another important factor affecting the substrate-induced response of the Tb(III)-centered luminescence in the aqueous colloids of the SNs. The Quenching of “Core”, “Core−Shell” and “Shell” SNs by Copper Ions. According to our previous results, the quenching effect of copper ions on the Tb(III)-centered luminescence of “core−shell” SNs is contributed by both dynamic and static mechanisms.27 Indeed, in the present work, both I/I0 and τ/τ0 values in the aqueous dispersions of the SNs depend on the concentration of the copper ions (Figure 5). The profiles of these dependencies are quite different for the studied SNs. The quenching of “core” SNs is manifested by a small decrease of the I/I0 values without any decrease of the τ/ τ0 values. This tendency is similar to the quenching results in the solutions of MC540, thus confirming the predominant location of the Tb(III) complexes within the core. The concentration dependences of the I/I0 and τ/τ0 values coincide in the “core−shell” and “shell” colloids within 0−0.01 mM of the Cu(II) ions (Figure 5), which proves the predominance of

Figure 5. I/I0 (1,3,5) and τ/τ0 (2,4,6) values of “core−shell” (1,2), “shell” (3,4), and “core” (5,6) SNs (0.028 g·L−1) at various concentrations of CuSO4 at pH 7.4. The SD values are designated by error bars.

the dynamic quenching mechanism in this concentration range. Further increase of the Cu(II) concentration reveals the contribution of the static mechanism to the quenching of both “core−shell” and “shell” SNs. Still, the obtained data (Figure 5) show the difference between these two types of SNs. The quenching of “core−shell” SNs is manifested by the decrease of both I/I0 and τ/τ0 values upon reaching the saturation level (Figure 5). The τ/τ0 values go through a minimum for the “shell” colloids of SNs, then increase to values close to 1 at 0.1 mM of CuSO4, while I/I0 values decrease to the saturation level similar to “core−shell” SNs. So, the dynamic quenching mechanism dominates in the concentration range 0−0.01 mM, while it comes to nothing at 0.01−0.1 mM of copper ions, with simultaneous significant quenching through the static mechanism. The uncommon dependence of τ/τ0 on the concentration of Cu(II) ions can be explained by the different depth of their diffusion into the silica matrix in various concentration conditions. The mostly dynamic quenching mechanism, observed in the Cu(II) concentration range of 0−0.01 mM, indicates an interfacial location of the quenching ions. The predominance of the static mechanism at the higher concentrations of the copper ions (0.01−0.1 mM) points to the quenching through the ion exchange (Scheme 2c) due to the deeper penetration of the Cu(II) ions into the silica matrix under these conditions. Nevertheless, the I/I0 values come to the saturation level at 0.04 mM of the quenching ions in the aqueous colloids of both “core-shell” and “shell” SNs, which points to the Tb(III) complexes, which are not available for the quenching by the copper ions. The observed tendencies also indicate the greater availability of the Tb(III) complexes at the silica/water interface for the ion exchange in the “core−shell” than in the “shell” colloids. This confirms the influence of the synthetic procedure (Scheme 1) on the percentage of the Tb(III) complexes at the silica/water interface. Effect of Phospholipid Deposition on the Substrate Response Ability of “Core”, “Core−Shell” and “Shell” SNs. Deposition of PC bilayer onto the SNs was performed through a common procedure, which is presented in detail in our previous report.38 The obtained results (Figure 6) show that the effect of PC deposition on steady state and timeresolved luminescence of the SNs also depends on their type. The luminescence of “core” SNs remains unchanged in the F

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Figure 6. (a) The I/I0 (1,3,5) and τ/τ0 (2,4,6) values of “core−shell” (1,2), “shell” (3,4) and “core” (5,6) SNs (0.028 g·L−1) at various concentrations of PC at pH 7.4. (b) The I/I0 (1,3) and τ/τ0 (2,4) values of “core−shell” SNs (0.028 g·L−1) in the presence of PC (0.004 g·L−1) at various concentrations of substrates: 1,2 - Na2EDTA, ATP GTP; 3,4 - AMP, GMP, CuSO4 at pH 7.4. The SD values are designated by error bars.

values is worth noting. These values remain unchanged (the I/ I0 values are close to 2) within the whole range of the concentrations in the solutions of copper ions, AMP, and GMP. This tendency is introduced in Figure 6b by one set of the experimental points for clarity. Doubly and triply charged anions, namely EDTA2−, ATP, and GTP, induce a peculiar effect on the I/I0 values. Indeed, even small concentrations of these anions (close to 0.01 mM) induce about 2-fold decrease of the I/I0 values (from 2 to 1). This tendency is very similar for these chelating anions and thus is represented in Figure 6b by one set of the experimental points for clarity (curve 1). The following question arises: What is the reason for the observed peculiar behavior of ATP, GTP and EDTA2−? The literature data highlight the importance of the electrostatic interactions in the binding ability of the nucleotides with phospholipids bilayers.42,43 In this connection, the work,41 introducing a route to sense the binding of phosphate anions, including AMP and ATP with DPPC (1,2-dipalmitoyl-snglycero-3-phosphocholine) vesicles through the quenching of Tb(III)-centered luminescence is worth noting, since it confirms the more tight binding of ATP versus AMP with DPPC bilayer. Thus, the significant decrease of the I/I0 values of “core-shell” SNs coated by the PC bilayers upon the increase of ATP and GTP concentrations correlates with their binding with the phospholipid bilayers. According to the above-mentioned assumption, the increase of the I/I0 values under the PC bilayer deposition onto “coreshell” SNs results from the dehydration of the Tb(III) complexes at the silica/PC interface. Thus, the effect of ATP, GTP, and EDTA2− on the I/I0 values can be explained by the reestablished hydration of the interfacial Tb(III) complexes. The reasons of this effect are not clear at the moment, nevertheless the results (Figure 6) once more highlight the Tb(III)-centered luminescence of “core−shell” SNs as the most responsive to the interactions at the silica/water interface. Hence, “core−shell” SNs, being used as the silica beads for the PC bilayers, provide a basis to sense the permeability of the latter. This proposed route is based on the Tb(III)-centered luminescence of the silica beads without the embedding of the dye or lanthanide labeled patches into the phospholipid bilayers.

wide concentration range of the PC vesicles, while the enhancement of the Tb(III)-centered luminescence is observed in the colloids of “core−shell” and “shell” SNs at a concentration of PC equal to 0.004 g·L−1. Nevertheless, the difference between the I/I0 and τ/τ0 values of “core−shell” and “shell” SNs is observed in the narrow concentration range (0− 0.01 g·L−1) of PC, while these values come to the same saturation level with a further increase of the PC concentration for both SNs (Figure 6a). The luminescence enhancement for “core−shell” SNs in the narrow concentration range of PC is partly contributed by a dynamic mechanism (Figure 6a). This result indicates the decreased contribution of radiationless decay as the cause of the observed luminescence enhancement. The nonradiative energy transfer to the high-frequency O−H oscillators greatly contributes to a radiationless decay of lanthanide complexes in aqueous solutions.3 Thus, the dehydration of the Tb(III) complexes at the silica/water interface under the PC bilayer deposition should be assumed as one of the reasons for the luminescence enhancement. The dehydration effect on the Tb(III)-centered luminescence is highlighted in previous work,12 where the luminescence enhancement results from the interaction of the Tb(III)-containing proteins with phospholipids. In this study, this assumption is confirmed by the lack of luminescence response on PC in the case of the “core” SNs (Figure 6a). The different response of “core−shell” and “shell” SNs provides one more confirmation for the synthetic procedure to affect the distribution of the Tb(III) complexes between the silica/water interface and the silica matrix. The quenching measurements in aqueous colloids of the SNs at various concentrations of the quenching ions were performed in order to reveal the effect of the PC bilayer on the luminescence response of the studied SNs. The lack of the more or less significant response of the Tb(III)-centered luminescence to the quenching ions is rather anticipated for the “core” and “shell” SNs, since their own response ability is insignificant (Figure S3 in the SI). A lack of any quenching is observed at various concentrations of copper ions, Na2EDTA, and nucleotides for the PC-coated “core−shell” SNs. This result indicates a low content of the quenching ions at the silica/PC interface due to their poor penetration through the PC bilayer. Nevertheless, some influence of the nature of the quenching ions on the I/I0 G

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(4) Schaferling, M.; Wolfbeis, O. S. Europium tetracycline as a luminescent probe for nucleoside phosphates and its application to the determination of kinase activity. Chem.Eur. J. 2007, 13, 4342−4349. (5) Weitz, E. A.; Chang, J. Y.; Rosenfield, A. H.; Pierre, V. C. A selective luminescent probe for the direct time-gated detection of adenosine triphosphate. J. Am. Chem. Soc. 2012, 134, 16099−16102. (6) (a) Mameri, S.; Charbonniere, L. J.; Ziessel, R. F. Lanthanide/ ATP interaction in water mediated by luminescent hemisphericalshaped complexes. Inorg. Chem. 2004, 43, 1819−1821. (b) Charbonniere, L. J.; Schurhammer, R.; Mameri, S.; Wipff, G.; Ziessel, R. F. Photophysical and structural impact of phosphorylated anions associated to lanthanide complexes in water. Inorg. Chem. 2005, 44, 7151−7160. (7) Liu, X.; Xu, J.; Lv, Y.; Wu, W.; Liu, W.; Tang, Y. An ATPselective, lanthanide complex luminescent probe. Dalton Trans. 2013, 42, 9840−9846. (8) Alam, A.-M.; Kamruzzaman, M.; Lee, S. H.; Kim, Y. H.; Jo, H. J.; Kim, S. H.; Park, S.-R. Sensitive determination of adenosine disodium triphosphate in soil, milk, and pharmaceutical formulation by enoxacineuropium (III) fluorescence complex in solution. J. Lumin. 2012, 132, 789−794. (9) Pal, R.; Parker, D.; Costello, L. C. A europium luminescence assay of lactate and citrate in biological fluids. Org. Biomol. Chem. 2009, 7, 1525−1528. (10) Atkinson, P.; Bretonniere, Y.; Parker, D. Chemoselective signalling of selected phospho-anions using lanthanide luminescence. Chem. Commun. 2004, 10, 438−439. (11) Azab, H. A.; Mogahed, E. M.; Awad, F. K.; Abd El Aal, R. M.; Kamel, R. M. Fluorescence and electrochemical recognition of nucleosides and DNA by a novel luminescent bioprobe Eu(lll)− TNB. J. Fluorescence 2012, 22, 971−992. (12) Gupta, S.; Mondal, S.; Mhamane, A.; Datta, A. Smart “lanthano” proteins for phospholipid sensing. Inorg. Chem. 2013, 52, 12314− 12316. (13) Härmä, H.; Sarrail, G.; Kirjavainen, J.; Martikkala, E.; Hemmilä, I.; Hänninen, P. Comparison of homogeneous single-label fluorometric binding assays: Fluorescence polarization and dual-parametric quenching resonance energy transfer technique. Anal. Chem. 2010, 82, 892−897. (14) Meskers, S. C. J.; Dekkers, H. P. J. M. Binding of vitamin B12 and B12a to an antibody and to haptocorrin probed by enantioselective quenching of Tb(pyridine-2,6-dicarboxylate)33− luminescence. J. Am. Chem. Soc. 1998, 120, 6413−6414. (15) Selvin, P. R.; Rana, T. M.; Hearst, J. E. Luminescence resonance energy transfer. J. Am. Chem. Soc. 1994, 116, 6029−6030. (16) Kessler, M. A. Probing the dissociation state of acid-base indicators by time-resolved lanthanide luminescence: A convenient transduction scheme for optical chemical sensors. Anal. Chem. 1999, 71, 1540−1543. (17) Fö r ster, Th. 10th Spiers Memorial Lecture. Transfer mechanisms of electronic excitation. Discuss. Faraday Soc. 1959, 27, 7−17. (18) Ye, Z.; Tan, M.; Wang, G.; Yuan, J. Development of functionalized terbium fluorescent nanoparticles for antibody labeling and time-resolved fluoroimmunoassay application. Talanta 2005, 65, 206−210. (19) Mustafina, A. R.; Fedorenko, S. V.; Konovalova, O. D.; Menshikova, A. Y.; Shevchenko, N. N.; Soloveva, S. E.; Konovalov, A. I.; Antipin, I. S. Novel highly charged silica-coated Tb(III) nanoparticles with fluorescent properties sensitive to ion exchange and energy transfer processes in aqueous dispersions. Langmuir 2009, 25, 3146−3151. (20) Enrichi, F.; Ricco, R.; Scopece, P.; Parma, A.; Mazaheri, A. R.; Riello, P.; Benedetti, A. Comparison of Eu(NO3)3and Eu(acac)3precursors for doping luminescent silica nanoparticles. J. Nanopart. Res. 2010, 12, 1925−1931. (21) Gao, F.; Luo, F.; Chen, X.; Yao, W.; Yin, J.; Yao, Z.; Wang, L. A novel nonenzymatic fluorescent sensor for glucose based on silica

CONCLUSIONS Summarizing, the present work highlights the impact of both energy transfer and inner sphere variations of the Tb(III) complexes doped into silica nanoparticles in the substrateinduced response of Tb(III)-centered luminescence. The distribution of the Tb(III) complexes within various zones of the silica nanoparticles, such as core, shell, and the silica/water interface, is revealed as the main factor affecting the Tb(III)centered luminescence response of the silica nanoparticles to substrates. An easy modification of the doping procedure is offered as a convenient route to obtain “core−shell”, “core”, and “shell” silica nanoparticles. The difference in the luminescence response of “core−shell” and “shell” SNs to chelating anions and copper ions highlights the effect of the synthetic conditions (Tb(III):TEOS ratio) on the interfacial location of the Tb(III) complexes. Thus, the luminescence of “core−shell” SNs is most responsive to any interfacial interactions, including a phosphatidylcholine (PC) adsorption. Quenching measurements in the aqueous colloids of the SNs coated by the PC bilayer reveal the impermeability of the latter toward the quenching ions. Nevertheless, the Tb(III)-centered luminescence of “core−shell” SNs coated by PC gives a selective response to ATP and GTP versus AMP and GMP. The dehydration/hydration of the Tb(III) complexes at the silica/water interface of “core−shell” SNs may be the reason for their particular response to both PC bilayer deposition and its binding ability toward the nucleotides. The highlighted regularities provide a basis for a widening of the applicability of silica nanoparticles doped with lanthanide complexes in sensing and imaging.

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ASSOCIATED CONTENT

S Supporting Information *

The emission spectra and DLS data of Tb(III)-doped silica nanoparticles are presented in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank RFBR (Project No. 13-03-00045-a) for financial support. The work of V.V.G. was performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.

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REFERENCES

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