Assessment of Submicron Particle Zeta Potential in Simple Electrokinetic

Electrophoresis 2019, 40, 1395–1399 1395 Samuel Hidalgo-Caballero1,2 Short Communication Cody Justice Lentz1 Blanca H. Lapizco- Encinas1 Assessment of submicron particle zeta 1Microscale Bioseparations potential in simple electrokinetic Laboratory and Biomedical Engineering Department, microdevices Rochester Institute of Technology, Rochester, NY, USA The present communication illustrates the use of simple electrokinetic devices for the assessment of the zeta potential of submicron polystyrene particles. A combination of 2Facultad de Ciencias Fısico´ Matematicas,´ Benemerita´ manual and automatic particle tracking was employed. This approach allows for charac- Universidad Autonoma´ de terizing particles in the same conditions and devices in which they can be separated, e.g. Puebla, Puebla, Mexico´ dielectrophoretic separations; making the resulting data readily applicable. Received October 9, 2018 Keywords: Revised November 8, 2018 Electrical charge / Electrokinetics / Electrophoresis / Submicron particles / Zeta Accepted November 21, 2018 potential DOI 10.1002/elps.201800425 Additional supporting information may be found online in the Supporting Infor- mation section at the end of the article. Particle migration is an important research area in microflu- White et al. [8] employed CE experimentation to determine idic devices, in particular, when working with electric field the ␨p of polystyrene particles in order to assess particle con- driven techniques, one crucial property is the particle zeta ductivity and predict dielectrophoretic behavior. Other stud- potential (␨p ). This parameter accounts for the electrical ies have been focused on characterizing the relationship be- charge present on a particle, as it characterizes the electrical tween electrophoretic migration and particle size [7, 9] with double layer (EDL) around the particle [1]. Particle zeta simultaneous determination of ␨p and zeta potential of the potential determines the electrophoretic mobility (␮EP)and channel surface (␨w) [1, 10]. Recognized research groups in the electrophoretic migration of a particle. Differences in the field of CE have dedicated considerable attention to the particle electrophoretic migration are widely exploited in measurement and prediction of ␮EP.TheGasˇ group devel- analytical electrokinetic separations, such as CE, capillary oped a sophisticated software package called PeakMaster for electrochromatography, isotachophoresis, isoelectric focus- the prediction of ␮EP of analytes of interest [11]. The Kasiˇ ckaˇ ing, and dielectrophoresis [2–4]. Moreover, most of the group has studied the determination of ␮EP for a wide array of techniques mentioned above can be enhanced by employing analytes [12]. Our research group has analyzed the electroki- liquid metal electrodes [5, 6] that provide a better control of netic migration of micron-sized particles and cells by using the electric field while avoiding the divergence that usually particle image velocimetry (PIV) [13,14]. The methodology re- appears in the case of thin electrode surfaces. ported here allows extending this approach, that only requires Characterizing the ␨p and ␮EP allows assessing particle simple devices, to the assessment of the ␨p of nanoparticles. surface and morphological properties, which can be related This study presents the experimental analysis of ␨p for to important biological attributes [7]. Furthermore, knowing 12 distinct types of submicron polystyrene particles (diam- the ␨p enables a better selection of the proper separation tech- eter from 100–500 nm). A combination of PIV and current nique to be used for a given particle sample or application. monitoring was employed to characterize particle electroki- Significant efforts have been devoted to the characterization netic migration in microchannels made from PDMS. Particle of ␨p and ␮EP for microparticles and nanoparticles [1, 7–10]. tracking was an essential step that had to be modified when assessing the smaller particles (100–200 nm) in our study, due to the diffraction limit for an optical microscope [15, 16]. ␨ Correspondence: Professor Blanca H. Lapizco-Encinas, Mi- This study demonstrates that the characterization of the p of croscale Bioseparations Laboratory, Rochester Institute of Tech- submicron particles is possible in simple microfluidic chan- nology, Institute Hall (Bldg. 73), Room 3103, 160 Lomb Memorial nels without the need of specialized equipment, such as a zeta Drive, Rochester, NY 14623, USA analyzer. The data generated in this study provides a valuable E-mail: [email protected] tool for designing new electric-field driven microfluidic sys- Abbreviations: EDL, electrical double layer; EK, electrokinetic; EO, electroosmosis; EP, electrophoresis; iDEP, insulator- based dielectrophoresis; PIV, particle image velocimetry Color online: See article online to view Figs. 1 and 2 in color. C 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com 1396 S. Hidalgo-Caballero et al. Electrophoresis 2019, 40, 1395–1399 Figure 1. (A) Schematic representation of the microchannel including the win- dow of study. For illustration purposes, a particle with negative charge is depicted in this image; therefore the EP migration is toward the inlet (left). Sample is introduced at the inlet, where the posi- tive electrode is located, the ground electrode is at the outlet. (B) Picture of a 500 nm red particle sample before application of an electric potential. (C) Trace lines of 500 nm red particles under an applied potential of 25 V. (D) Particle velocity as function of the electric field for particles 300–500 nm diameter. Legend: 320 nm particles (continuous line) and 500 nm particles (dashed line). tems for particle analysis and separation. In particular, our EP is the movement of particles relative to the suspension research group works with insulator-based dielectrophoresis medium, under the influence of an electric field. (iDEP) that combines electrophoresis (EP), electroosmosis According to the Helmholtz-Smoluchowski Equation [18] (EO), and DEP for the manipulation of nano and microparti- the electroosmotic velocity is given by: cles, including bioparticles such as DNA, proteins, virus and ε ␨ v = ␮ =− m w cells. Additionally, the results from this study are essential for EO EO E ␩ E (1) any type of computational modeling or applications used for ␮ ε predicting the behavior of particles in microscale electroki- where EO is the electroosmotic mobility, m is the media per- mittivity, ␨w is the wall zeta potential, ␩ is the media viscosity netic (EK) systems, such as iDEP devices. It is expected that similar assessments will be carried out for the characteriza- and E is the local electric field. For particles, the Helmholtz- tion of the zeta potential of biological submicron particles. Smoluchowski Equation is valid when the particle radius is Electrokinetic phenomena are a consequence of a polar- much greater than the Debye length (␭D). The other limit ization mechanism at the interface between a solid and a is the Huckel¨ approximation which is valid when the parti- liquid which creates an EDL [17]. The first layer (Stern layer) cle radius is equal to or smaller than ␭D. None of these two comprises the surface charge generated by ion adsorption on limits are the case for the particles in this study. Taking into the solid surface itself. The second layer (Helmholtz layer) account the low ionic strength of the suspension medium, is composed of mobile counterions diffusely attracted to the the Debye length under these conditions was estimated as ␭ = . ␬ = ␭−1 surface charge that give rise to two important phenomena: D 6 1 nm ( D ); considering a as the particle radius, EO and EP. Electroosmosis is the motion of a fluid, while this produces ␬a values between 8 and 41 for the particles C 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com Electrophoresis 2019, 40, 1395–1399 Miniaturization 1397 Ta b l e 1 . Properties of the 12 fluorescent polystyrene particles analyzed in this study, including mobility (␮EK and ␮EP), particle zeta 2 potential (␨p), and fitting correlation (R ) values for the vEK versus electric field data −10 2 Sample Particle Color Brand Surface ␮EK ±1 × 10 ␮EP ±1 × ␨p ±2 R values 2 −1 −1 -9 2 -1 -1 size (nm) functionality (m V s ) 10 (m V s ) (mV) vEK vs. E 1 500 Red Invitrogen Carboxyl 1.6 ±0.01 × 10−8 –6.0 × 10−8 –81 0.9984 2 500 Green Magsphere Carboxyl 1.1 × 10−8 –6.5 × 10−8 –87 0.9940 3 500 Red Magsphere Carboxyl 1.8 × 10−8 –5.8 × 10−8 –78 0.9775 4 320 Green Magsphere Carboxyl 1.2 × 10−8 –6.3 × 10−8 –88 0.9997 5 200 Green Invitrogen Carboxyl –4.4 × 10−9 –8.0 × 10−8 –116 0.9414 6 200 Orange Invitrogen Carboxyl –1.3 × 10−8 –8.9 × 10−8 –128 0.9868 7 200 Red Invitrogen Amine 2.6 × 10−8 –0.9 × 10−8 –72 0.9916 8 200 Red Magsphere Amine 2.4 × 10−8 –5.1 × 10−8 –72 0.9960 9 100 Red Invitrogen Carboxyl 7.8 × 10−9 –6.8 × 10−8 –106 0.9862 10 100 Green Invitrogen Carboxyl 9.4 × 10−9 –6.6 × 10−8 –104 0.9952 11 100 Green Magsphere Carboxyl 4.7 × 10−9 –7.1 × 10−8 –111 0.9921 12 100 Green Magsphere Amine 8.3 × 10−9 –6.8 × 10−8 –106 0.9979 An analysis of error estimation for the ␨pvalues, which was calculated as ±2 mV, is included in Section 4 of the Supporting Information. in this study (100–500 nm diameter). For these intermediate particles with diameters from 100–500 nm were studied (Ta- conditions [8], the function proposed by Henry [19], for which ble 1). Particle suspensions ranged in concentration from Ohshima developed an approximation [20] applies: 3.4–7.3 × 108 particles/mL. The suspending media was made ε from DI water with 0.05% v/v Tween 20, to reduce particle 2 m␨ p ␮EP = f (␬a) (2) adhesion, and addition of 0.1 M KOH, to produce a pH of 3␩ 6.0–6.5 and a conductivity of 25.3 ± 0.1 ␮S/cm.

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