Electrokinetic Manipulation of Silver and Platinum Nanoparticles and

Letter pubs.acs.org/ac

Electrokinetic Manipulation of Silver and Platinum Nanoparticles and Their Stochastic Electrochemical Detection Jason Bonezzi, Tulashi Luitel, and Aliaksei Boika* Department of Chemistry, The University of Akron, 190 East Buchtel Common, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Electrokinetic phenomena such as dielectrophoresis and electrothermal fluid flow are used to increase the rate of mass transfer of silver and platinum nanoparticles and improve their stochastic electrochemical detection. These phenomena are induced by applying a high frequency alternating current (ac) waveform between a counter electrode and a working disk microelectrode. By recording chronoamperograms at room temperature and various ac powers, it is shown that the ac heating leads to an increase in the collision frequency of studied nanoparticles with working electrode surface by a factor of ∼101−103 as well as the increase in the magnitude of the measured faradaic response. It is suggested that the developed methodology could be used in the future to improve the detection of ultralow concentrations of various important bioanalytes.

he field of stochastic electrochemical detection of individual analyte entities has seen tremendous progress during the past decade.1 The palette of species that are amenable to the sensitive detection based on their collisions with an indicator electrode surface includes hard and soft nanoparticles (NPs) such as Pt,2,3 Ag,4−6 Au,7,8 IrOx,9 magnetite,10 Ni,11 Cu,12 polymer beads,13,14 emulsions,15 indigo,16 numerous biological macromolecules such as enzymes, proteins, antibodies and DNA,17 vesicles,18 cancer cells,19 viruses,20−22 and bacteria,23,24 and even single ions.25,26 Undoubtedly, more reports concerning the electrochemical detection of novel analytes will keep appearing in the literature in the future. Yet, there is another issue that needs addressing in order to be able to detect ultralow concentrations of various species. For detection of individual analytes to work, the size of the indicator electrode (or sensor) needs to be very small and in some cases comparable to the size of the analyte species. This, however, leads to a challenge since one needs to ensure that the analyte finds the electrode to be detected there. Most reported approaches rely on diffusion as the driving force; however, the diffusional mass transfer is random in a sense that each step the analyte makes is in an arbitrary, random direction. Therefore, the measured analyte concentrations are typically in the picomolar (pM, 10−12 M) concentration range, and the rate of collisions is a function of the diffusion coefficient of the species. By employing a deterministic force such as the Coulomb force, under the condition of electrophoretic migration, one can detect even large and otherwise slowdiffusing analytes such as polystyrene and silica particles down to the femtomolar (fM, 10−15 M) concentration range.14,27,28 Another approach is to utilize preconcentration using magnetic force.29 However, these two methodologies are not ideal, as the first one requires that the detection is done under the condition

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of low supporting electrolyte concentration, while the second relies exclusively on the use of magnetic particles. Electrokinetic phenomena such as dielectrophoresis (DEP)30,31 and electrothermal fluid flow (ETF)32,33 have not been previously considered for preconcentration of analytes in combination with their stochastic electrochemical detection. However, they have been known for over 60 years, and there are numerous reports describing the use of DEP, ETF, and electroosmotic flow (EOF) for manipulation and sorting of various particles such as biological cells, bacteria, viruses, proteins, DNA and RNA macromolecules, latex and other polymer beads, and metal NPs.30,32,34−40 In this paper, we are describing the use of DEP and ETF for manipulation of Ag and Pt NPs, which results in the NP accumulation at the electrode and their electrochemical detection. In the past, the aforementioned electrokinetic phenomena have been observed and investigated at disk microelectrodes polarized by a highfrequency alternating current (ac) waveform.41−46 It has been shown that the ac modulation does not lead to any substantial increase in the noise in the system, since it is filtered out by a low-pass filter, and the usual dc voltammetric experiments can be easily performed under the conditions of elevated temperature and high rates of mass transfer. Our goal here is to demonstrate the feasibility of stochastic electrochemical detection performed in combination with the electrokinetic manipulation of analytes by the DEP and ETF phenomena. Received: July 18, 2017 Accepted: August 7, 2017 Published: August 7, 2017 8614

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Figure 1. Chronoamperometric curve obtained in an experiment involving the detection of Ag NPs (113 nm diameter) in 100 mM citrate buffer (pH 3.8). Working electrode: 11 μm carbon disk, applied potential +0.5 V vs Ag|AgCl. Ag NP concentration: 50 fM. Particles were injected at t = 200 s, and at t = 300 s the ac waveform was applied (frequency 102 MHz, power 16−24.4 dBm). The results of data analysis (average oxidation charge, Qs, and frequency of collisions, f) for each observation period are included in the graph.

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under reflux for 1 h. The flask was then removed and cooled in an ice bath. The initial procedure gave Ag nanoparticles with an average diameter of 88.6 ± 2.1 nm (measured by Malvern Nanosight NS500 instrument, as discussed below). These particles were used in a subsequent synthesis to give particles with an average diameter of 112.7 ± 14.5 nm and concentration of 3.325 × 1010 particles/mL. The growth of NPs was achieved by adding 100 μL of the seed particle to 25 mL of the 0.01 M AgNO3 solution, in 225 mL of water as it began to boil. The solution was boiled for 1 h under reflux before allowing it to cool in an ice bath. Before use, the particles were washed twice by centrifuging 1.5 mL portions at 14 000 rpm for 20 min, removing the liquid with a pipet, and redispersing them in 1.5 mL of water. Synthesis of platinum nanoparticles has been performed in two steps: first, seed NPs (∼4 nm in diameter) were prepared, which were afterward used to produce bigger NPs (∼33 nm). All glassware was cleaned in aqua regia for 1 h, washed successively with water, acetone, ethanol, and 2-propanol for at least five times, and then left to dry overnight. The synthesis method has been adopted from refs 27 and 48: 20 mL of nanopure water was boiled under constant stirring in a 25 mL round-bottom flask fitted with a condenser; 778 μL of 3.8 mM H2PtCl4·6H2O (chloroplatinic acid) was added to the boiling water. After 1 min, 244 μL of a solution containing 38 mM trisodium citrate and 2.6 mM citric acid was injected at once into the boiling solution. Without further waiting, 122 μL of 21 mM sodium borohydride solution was mixed into the boiling solution. Within 10−60 s, the colorless solution turned dark thus showing the evidence of formation of citrate-capped platinum seed particles. To synthesize 33 nm Pt NPs, 1 mL of platinum seed solution just prepared was mixed with 29 mL of nanopure water and was stirred continuously in a flask. A volume of 45 μL of 0.1 M H2PtCl4·6H2O was injected into this solution, which was rested on a heating mantle. A volume of 500 μL of a solution containing 34 mM trisodium citrate and 71 mM L-ascorbic acid was added to the stirred solution. After fitting the reflux condenser to the flask, the temperature of the

EXPERIMENTAL SECTION Materials and Electrochemical Cell. All solutions were prepared using a Millipore Integral 5 water purification system. ACS-grade chemicals were purchased from Sigma-Aldrich and were used without further purification. The electrochemical cell included an Ag|AgCl|KCl (2 M) reference electrode (purchased from CH Instruments, Austin, TX), a counter electrode made of 0.3 mm platinum wire, and a working electrode. In experiments with Ag NPs, either an 11 μm carbon disk (BASi) or a custom-built 10 μm gold microelectrode was used as a working electrode. In experiments with Pt NPs, a 10 μm gold microelectrode was used. Fabrication of gold microelectrodes was done according to a well-established procedure.42 Instrumentation. The instrumental setup used in the experiments consisted of a commercially available potentiostat (model 760E, CH Instruments, Austin, TX) interfaced with an MXG analog signal generator (Keysight Technologies model N5181B), a low-pass filter and the electrochemical cell described above. The design of the filter was the same as reported in a previous publication from our group.46 The ac signal generator had nominal power output levels from −144 dBm to +26 dBm (at 1 GHz) and the frequency range from 9 kHz to 3 GHz. The ac waveform was applied between the counter and working electrodes. The electrochemical cell and the filter were positioned inside a grounded Faraday cage in order to minimize the effect of an external electromagnetic interference on experimental results. Methods. Silver nanoparticles were synthesized following the procedure outlined in Campbell et al.47 All glassware was washed with Alconox detergent, rinsed with copious amounts of water, and dried in a drying oven prior to synthesis. First, 25 mL of a 0.01 M AgNO3 solution was added to 225 mL of water in a 500 mL round-bottom flask. The solution was brought to a boil under reflux and vigorous magnetic stirring on a hot plate. A trisodium citrate solution was prepared by dissolving 1 g of the citrate salt in 100 mL of water. When boiling began, 5 mL of the citrate solution was added to the flask and allowed to boil 8615

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Analytical Chemistry heating mantle was raised gradually to 100 °C at a 5 °C/min rate. Within 25−40 min, faint yellowish color changed to black revealing the formation of bigger platinum nanoparticles. Boiling was continued for 1 h to complete the reaction. Characterization of NPs. Both Ag and Pt NPs were characterized using the Nanosight NS500 instrument from Malvern, USA. Nanosight’s built-in software was used to track NPs and determine their concentration and diameter, see Figures S1 and S2 in the Supporting Information for the sample data. The instrument was calibrated using 100 nm polystyrene bead solution provided by the vendor. Optical photomicrographs showing accumulation of Ag NPs on an electrode surface were obtained using an Olympus BX43 microscope equipped with a Qimaging Go-3 camera.

negative DEP. Since the permittivity of metal particles is essentially infinite, the value of the CMF is one, so the particles get pulled by the DEP force and accumulate in regions where the electric field is the strongest, i.e., the surface of the microdisk. Our current understanding is that the particles are carried by the ETF from the bulk solution (where the DEP force is relatively weak, due to FDEP ∝ ∇(∇φ)2) toward the surface of the electrode, where they are further preconcentrated by positive DEP. This view of the Ag NP manipulation by electrokinetic phenomena will need to be confirmed by theoretical simulations, which will be the matter of our future publication. The charge associated with oxidation of Ag NPs, under the conditions in Figure 1, has been determined by integration of the corresponding individual chronoamperometric peaks. The average values (Qs) are listed in Figure 1 and charge distributions are shown in Figure S10. One can see that, as the power of the applied ac signal increases, the charge due to oxidation of silver also increases. Statistical analysis of the data (Figure S10) indicates that there is a significant difference between the mean values of charge, Qs, obtained at 22−23.8 dBm and 24.4 dBm at the 95% confidence level (results for 16−20.8 dBm were not analyzed since a very limited number of collisions was obtained, less than 10, in each case). At the largest ac power used (24.4 dBm), the magnitude of the average charge was 4.60 pC, which is close to the theoretical value of 7.05 pC expected for complete oxidation of 113 nm Ag NPs based on the following equation:

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RESULTS AND DISCUSSION In Figure 1, one can see the results of a typical collision experiment involving electrochemical detection of Ag NPs. The experiment was performed under the conventional roomtemperature conditions as well as under application of an ac waveform with a frequency 102 MHz. The choice of this value of the frequency was dictated by the fact that it offers the most effective heating of the electrolyte solution, see Figure S3 in the Supporting Information. At room temperature (the period from 200 to 300 s on the It-curve in Figure 1), no collisions were observed, due to a low concentration of particles, 50 fM, and their relatively large diffusion coefficient. From a different experiment (Figure S4 in the Supporting Information) we determined that the expected frequency of collisions for the room-temperature conditions in Figure 1 is 0.0002 s−1. This value is too low to expect any fruitful collision during the 100 s observation time. However, as the power of the ac waveform is increased, the frequency of collisions increases substantially and culminates at a value of 0.99 s−1 (since we stopped further increase in the power of the ac signal). These results illustrate more than a 4 000-fold increase in the frequency of collisions of Ag NPs due to the application of a high frequency ac waveform. It should be noted that in a control experiment (Figure S8), no such behavior has been observed. The phenomena that are responsible for an increase in the frequency of collisions are, most likely, the combination of DEP and ETF; these phenomena were previously reported for ac modulated microelectrodes.42 Considering the increase in the faradaic current at the frequency of applied ac modulation (102 MHz), as shown in Figure S3, suggests the presence of the ETF. DEP of Ag NPs occurs because the NPs are easily polarized by the ac electric field. The magnitude of the DEP force is given by eq 1. FDEF = 2πa3ε0εmK ∇(∇φ)2

3 neF ⎛ rNP ⎞ ⎜ ⎟ Q= NA ⎜⎝ rAg ⎟⎠

(2)

where ne is the stoichiometric number of electrons involved in the half-reaction of silver oxidation (1), F is Faraday’s constant (96 485 C mol−1), NA is Avogadro’s number (6.02 × 1023 mol−1), rNP is the particle’s radius, and rAg is the radius of a silver atom (0.160 nm). The incomplete oxidation of Ag NPs, represented by “bunches” of collision peaks and the values of Q significantly smaller than what is expected according to eq 2, has been recently reported by several groups.5,6 The argument for this phenomenon is that the NPs, undergoing random walk, have a finite probability to leave the electrode thus not being completely oxidized (also, the formation of the Ag+-ions at the surface of the NP contributes to the NP repulsion from the positive electrode). The situation in our experiments is different since the applied ac waveform leads to polarization of Ag NPs and the resultant DEP force on them. In a different experiment, using optical microscopy, we observed increased accumulation of Ag NPs at the disk electrode circumference with an increase in the ac amplitude (see Figure 2). These results also indicate that not all NPs are getting completely oxidized, if any. An explanation to the incomplete oxidation could be that it is due to the contact resistance and the ohmic drop developing as the particles are getting attached under the action of the DEP force. Also, there could be kinetic limitations to Ag NP oxidation.5 Investigations of these possible issues are planned for our future work. Interestingly, the results in Figure 2 show that the NPs arrange in chains under the action of electrokinetic forces similarly to a number of previous observations.49,50 An explanation to this phenomenon is that the local electric field around an attached NP is stronger due to its nanoscale surface

(1)

where the term K represents the real part of the ClausiusMossotti factor (CMF), K = Re[(εp − εm)/(εp + 2εm)]; it dictates the direction of the DEP. Particle radius is given by a, ε0 is the permittivity of free space, εm is the dielectric permittivity of the medium (electrolyte solution), εp is the dielectric permittivity of the particle. The field factor ∇(∇φ)2 represents the effect of the electric field strength, ∇φ. For positive values of the CMF, the DEP force is directed toward the stronger electric field. Thus, the positive DEP of the particles is observed. On the other hand, if the CMF is negative, the opposite is observed and the phenomenon is termed the 8616

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(Figures S5 and S7). These charge values, compared to the expected charge of 3.6 pC, according to eq 2, suggest that the particles colliding with the electrode are oxidized completely. Results similar to those in Figure 1 have also been observed for another extensively studied collision system involving the detection of Pt NPs on a gold microelectrode and utilizing the hydrazine oxidation reaction (electrocatalytic amplification method3,27,51). In Figure 3 one can see the results of such experiments (for control experiment, see Figure S9). Here, under room temperature conditions (time 100−400 s), the frequency of collisions is only 0.017 s−1 since the concentration of Pt NPs is rather small and their diffusion coefficient is relatively small too. However, as the ac waveform is applied, the frequency of collisions increases with the power of the ac signal, and at the end of the experiment reaches the value (0.173 s−1) 1 order of magnitude higher than the initial frequency. This result can be similarly explained by the action of the DEP and ETF phenomena. We have also investigated the change in the current step magnitude, as shown in Figure 3 and Figure S11. One can see that the average current step magnitude, Is, increases from 0.133 nA to 0.254 nA. The statistical analysis (comparison of the means) reveals that this change is not significant at 95% confidence level. The explanation could be that the ac heating is not very strong at the power levels used (the data in Figure S9 shows only a small increase in the faradaic current for hydrazine oxidation upon application of an ac waveform). In addition, it is known that Pt NPs aggregate in the presence of hydrazine;52,53 this could further explain a relatively high scatter of current steps, which affects the statistical analysis. Yet, a clear and significant increase in the frequency of collisions in Figure 3 confirms a positive effect of electrokinetic phenomena on the mass transfer of the NPs. One of the remaining challenges of the analytical approach proposed here is to be able to measure the temperature achieved at the working electrode due to ac heating. Currently, we assess the presence of heating by the change in the faradaic current of water oxidation, as shown in Figure S3. We are

Figure 2. Photomicrographs showing accumulation of Ag NPs as a function of the power of applied ac waveform at frequency 100 MHz. Chronoamperograms (not shown) were recorded for 300 s under the conditions identical to those in Figure 1. Particle concentration, 2 pM.

curvature. Thus, the NPs responding to the electric field tend to attach to each other and form chain-like structures. In an additional experiment, as the power of ac waveform has been increased further to 26 dBm, we observed complete oxidation of ∼90 nm Ag NPs. This is evidenced by the absence of Ag NPs accumulation (see Figure S6 for images of the electrode surface), while multiple oxidation spikes have been detected, as shown in Figure S7 in the Supporting Information. In addition, the average oxidation charge has increased to 3.3 pC compared to the 1.7 pC for the results obtained at 16 dBm

Figure 3. Chronoamperometric curve obtained in an experiment involving the detection of Pt NPs (33 nm diameter) in 15 mM hydrazine, 50 mM sodium phosphate buffer solution (pH 7.5). Working electrode: 10 μm gold disk, applied potential +0.05 V vs Ag|AgCl. Pt NP concentration: 1 pM. Particles were injected at t = 100 s, and at t = 400 s the ac waveform was applied (frequency 97 MHz, power 9−13.8 dBm). The results of data analysis (average current step, Is, and frequency of collisions, f) for each observation period are included in the graph. 8617

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Analytical Chemistry working on better alternatives and will describe our findings in due course.

(4) Zhou, Y. G.; Rees, N. V.; Compton, R. G. Angew. Chem., Int. Ed. 2011, 50, 4219−4221. (5) Oja, S. M.; Robinson, D. A.; Vitti, N. J.; Edwards, M. A.; Liu, Y.; White, H. S.; Zhang, B. J. Am. Chem. Soc. 2017, 139, 708−718. (6) Ustarroz, J.; Kang, M.; Bullions, E.; Unwin, P. R. Chem. Sci. 2017, 8, 1841−1853. (7) Zhou, H.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. Lett. 2010, 1, 2671−2674. (8) Bentley, C. L.; Kang, M.; Unwin, P. R. J. Am. Chem. Soc. 2016, 138, 12755−12758. (9) Kwon, S. J.; Fan, F.-R. F.; Bard, A. J. J. Am. Chem. Soc. 2010, 132, 13165−13167. (10) Tschulik, K.; Haddou, B.; Omanović, D.; Rees, N. V.; Compton, R. G. Nano Res. 2013, 6, 836−841. (11) Zhou, Y. G.; Haddou, B.; Rees, N. V.; Compton, R. G. Phys. Chem. Chem. Phys. 2012, 14, 14354−14357. (12) Haddou, B.; Rees, N. V.; Compton, R. G. Phys. Chem. Chem. Phys. 2012, 14, 13612−13617. (13) Quinn, B. M.; van ’t Hof, P. G.; Lemay, S. G. J. Am. Chem. Soc. 2004, 126, 8360−8361. (14) Boika, A.; Thorgaard, S. N.; Bard, A. J. J. Phys. Chem. B 2013, 117, 4371−4380. (15) Kim, B. K.; Boika, A.; Kim, J.; Dick, J. E.; Bard, A. J. J. Am. Chem. Soc. 2014, 136, 4849−4852. (16) Cheng, W.; Zhou, X. F.; Compton, R. G. Angew. Chem., Int. Ed. 2013, 52, 12980−12982. (17) Dick, J. E.; Renault, C.; Bard, A. J. J. Am. Chem. Soc. 2015, 137, 8376−8379. (18) Lebègue, E.; Anderson, C. M.; Dick, J. E.; Webb, L. J.; Bard, A. J. Langmuir 2015, 31, 11734−11739. (19) Dick, J. E. Chem. Commun. 2016, 52, 10906−10909. (20) Dick, J. E.; Hilterbrand, A. T.; Boika, A.; Upton, J. W.; Bard, A. J. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5303−5308. (21) Dick, J. E.; Hilterbrand, A. T.; Strawsine, L. M.; Upton, J. W.; Bard, A. J. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 6403−6408. (22) Sepunaru, L.; Plowman, B. J.; Sokolov, S. V.; Young, N. P.; Compton, R. G. Chem. Sci. 2016, 7, 3892−3899. (23) Lee, J. Y.; Kim, B.-K.; Kang, M.; Park, J. H. Sci. Rep. 2016, 6, 30022. (24) Sepunaru, L.; Tschulik, K.; Batchelor-McAuley, C.; Gavish, R.; Compton, R. G. Biomater. Sci. 2015, 3, 816−820. (25) Dick, J. E.; Bard, A. J. J. Am. Chem. Soc. 2015, 137, 13752− 13755. (26) Dick, J. E.; Bard, A. J. J. Am. Chem. Soc. 2016, 138, 8446−8452. (27) Park, J. H.; Boika, A.; Park, H. S.; Lee, H. C.; Bard, A. J. J. Phys. Chem. C 2013, 117, 6651−6657. (28) Boika, A.; Bard, A. J. Anal. Chem. 2014, 86, 11666−11672. (29) Yoo, J. J.; Anderson, M. J.; Alligrant, T. M.; Crooks, R. M. Anal. Chem. 2014, 86, 4302−4307. (30) Pohl, H. A. Dielectrophoresis: The Behavior of Neutral Matter in Nonuniform Electric Fields;Cambridge Monographs on Physics; Cambridge University Press: Cambridge, U.K., 1978; 579 pages. (31) Pohl, H. A.; Hawk, I. Science 1966, 152, 647−649. (32) Morgan, H.; Green, N. G. AC Electrokinetics: Colloids and Nanoparticles; Research Studies Press: Baldock, England, 2003; p xvi, 324 pages. (33) Gonzalez, A.; Ramos, A.; Morgan, H.; Green, N. G.; Castellanos, A. J. Fluid Mech. 2006, 564, 415−433. (34) Hughes, M. P. Biomicrofluidics 2016, 10, 032801. (35) Jesus-Perez, N. M.; Lapizco-Encinas, B. H. Electrophoresis 2011, 32, 2331−2357. (36) Lapizco-Encinas, B. H.; Rito-Palomares, M. Electrophoresis 2007, 28, 4521−4538. (37) Demircan, Y.; Ozgur, E.; Kulah, H. Electrophoresis 2013, 34, 1008−1027. (38) Dash, S.; Mohanty, S. Electrophoresis 2014, 35, 2656−2672. (39) Jubery, T. Z.; Srivastava, S. K.; Dutta, P. Electrophoresis 2014, 35, 691−713. (40) Camacho-Alanis, F.; Ros, A. Bioanalysis 2015, 7, 353−371.

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CONCLUSIONS In this paper we report, for the first time, on the use of electrokinetic phenomena such as dielectrophoresis and electrothermal fluid flow for manipulation of Ag and Pt NPs in combination with their detection based on methods of stochastic electrochemistry. Stochastic electrochemical methods allow for an unprecedented sensitivity of analysis of single analyte entities, i.e., detection of one analyte at a time. Dielectrophoresis and electrothermal fluid flow induced by application of an ac waveform lead to intensification of mass transfer and, therefore, allow for detection of much lower concentrations of analytes that would otherwise be impossible, considering mass transfer by diffusion only. Therefore, the methodology introduced here is important for development of analytical schemes for analysis of ultralow concentrations of not only metal nanoparticles but also various bioanalytes such as bacteria, viruses, proteins, nucleic acids, and other disease pathogens and indicators.

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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.7b02807. Results of analyses of Ag and Pt NPs using Nanosight; example of determination of the optimum ac heating frequency; dependence of the frequency of collisions of Ag NPs on their concentration; optical photomicrographs showing accumulation (and the absence of it) of Ag NPs; additional chronoamperograms corresponding to the oxidation of Ag NPs; results of the control experiments; and the histograms showing the distribution of the data presented in Figures 1 and 3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (330) 972-6085. ORCID

Aliaksei Boika: 0000-0001-8249-0741 Author Contributions

A.B. designed the experiments. J.B. performed the experiments involving Ag NPs. T.L. performed the experiments with Pt NPs. All authors contributed to the writing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support to A.B. in the form of startup funding provided by The University of Akron is gratefully acknowledged.

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REFERENCES

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