Ion Concentration Polarization Effect Research Articles

Engineering Multi-field-coupled Synergistic Ion Transport System Based on the Heterogeneous Nanofluidic Membrane for High-Efficient Lithium Extraction

HighlightsThe first construct of a multi-field-coupled synergistic ion transport system (MSITS) in Li+ extraction is proposed.Effectively suppress the ion concentration polarization effect of the ion-enrichment zone at the membrane interface.The MSITS equipped with heterogeneous membrane exhibited outstanding separation performance with Li+ flux of 367.4 mmol m−2 h−1 and Li+/Co2+ selectivity of 216,412, outperforming previous reports.

Janus Metal–Organic Framework Membranes Boosting the Osmotic Energy Harvesting

The unique ion-transport properties in nanoconfined pores enable nanofluidic devices with great potential in harvesting osmotic energy. The energy conversion performance could be significantly improved by the precise regulation of the "permeability-selectivity" trade-off and the ion concentration polarization effect. Here, we take the advantage of electrodeposition technique to fabricate a Janus metal-organic framework (J-MOF) membrane that possesses rapid ion-transport capability and impeccable ion selectivity. The asymmetric structure and asymmetric surface charge distribution of the J-MOF device can suppress the ion concentration polarization effect and enhance the ion charge separation, exhibiting an improved energy harvesting performance. An output power density of 3.44 W/m2 has been achieved with the J-MOF membrane at a 1000-fold concentration gradient. This work provides a new strategy for fabricating high-performance energy-harvesting devices.

Enhanced immunoassay in a nanofluidic preconcentrator utilizing nano-interstices among self-assembled gold nanoparticles.

In this study, we developed a microfluidic device for a dual work of protein preconcentration and subsequent capture by an immunoassay system. The fabrication of nano-interstices (nanochannels) to generate ion concentration polarization effect (ICP) for the preconcentration was simply performed by exploiting the loose association of glass-on-modified AuNPs to the polydimethylsiloxane (PDMS) channel pads as well as spaces between the substrate and the PDMS pad caused by these AuNPs. As a result, 65-fold concentration enhancement was achieved when performed on a sample of fluorescein isothiocyanate-labeled bovine serum albumin (FITC-BSA). Furthermore, a modification procedure of antibodies responsible for capturing target proteins was performed on gold electrodes integrated into the proposed chip. After preconcentration, the immunoassay system was worked, and showed a good performance in capturing targets. Through this study, we demonstrated that the device can work efficiently for the dual purpose, has the potential to apply widely for the analysis and capture of various targets.

Simultaneous enrichment and separation based on ion concentration polarization effect on a paper based analytical device

In this paper, simultaneous enrichment and separation of ions and amphoteric components were successfully demonstrated by using electric field (E) and pH gradient (double gradient) in the ion depletion zone of anion concentration polarization interface established on a paper fluid channel. Experimental results were visualized with colored ions (bright blue and amaranth) and protein probes (phycocyanin and cytochrome C). With optimization, colored phycocyanin and bovine hemoglobin with similar pI as that of albumin and immunoglobulin respectively were well separated in 900 s with 10-fold enrichment effect. Based on the separation and enrichment function of this paper-based analytical device (PAD) and rapid selective staining of human serum albumin (HSA) with bromophenol blue, a rapid colorimetric detection of HSA was implemented with smartphone camera. A limit of detection (LOD) of 5.2 mg·L−1 was achieved in the clinically significant range of 10–300 mg·L−1 (R2 = 0.99). This method was applied to real human urine samples with good agreement (ɑ = 0.01) to clinical detection method (immunoturbidimetry). With the separation and enrichment functions of PAD, both the specificity and sensitivity were enhanced, which provides a solid basis for point-of-care test of microalbuminuria. Therefore, this PAD device is potential for sample pretreatment and detection of target components from complex physiological samples.

Impacts of transmembrane pH gradient on nanofluidic salinity gradient energy conversion

Solution pH can impact the nanochannel surface charge density, thus, to regulate the transmembrane ion transportation. Here, the performance of nanofluidic salinity gradient energy conversion is investigated, where the solution pH at the high/low concentration side varies separately, rendering asymmetric pH configurations. Results reveal that the solution pH at the low concentration side (pHL) exhibits a significant impact on the salinity gradient energy conversion. When pHL < Isoelectric Point (IEP), the energy conversion indicators are most stable under various solution pH at the high concentration side (pHH). The surface charge density of the nanochannel at the low concentration end determines the transmembrane ion transportation characteristics, dominating the energy conversion performance. The energy conversion performance via nanochannels of different lengths under asymmetric pH configurations is also studied. There exists optimal nanochannel length corresponding to the maximum electric power, which differs under various pH configurations. For longer nanochannels, the ion concentration polarization effect is weakened, allowing for a more effective energy extraction performance via pH-adjusted salinity gradients. At the length of 2000 nm, the electric power and energy conversion efficiency under the co-direction configuration is 78% and 97.9%, respectively, larger than those under the reverse direction configuration.

Unraveling the anomalous channel-length-dependent blue energy conversion using engineered alumina nanochannels

Blue energy conversion, where the chemical energy stored in salinity gradients can be converted into electricity with ion-selective nanochannel membranes, has considered to be one of the most promising renewable energies. Conventional understanding on this energy suggests that as to largely reduce the resistance, ultrashort channel membranes are required to gain high-energy output. To understand the channel-length-dependent blue energy conversion in detail, we engineered a series of highly ordered and uniform ~23.0 nm in diameter alumina nanochannel membranes with various lengths. Most anomalously, our experiments however show that for sufficiently short nanochannels, the shorter the channel length, regardless of surface charge nature, the smaller the generated power, violating the past understanding. The anomalous channel-length-dependent blue energy conversion is well supported by our rigorous model. The modeling reveals that ultrashort nanochannels will induce the significant ion concentration polarization effect, which appreciably undermines effective salinity ratio and ion selectivity in the nanochannel. If this effect dominates, the nanofluidic osmotic power turns into a decrease with decreasing channel length. Both the experimental and theoretical results reported consistently highlight the importance of osmotic ion transport especially in ultrashort nanochannels, and this finding shed light on the design of high-efficiency blue energy harvesters. • Alumina nanochannel membranes with highly ordered channel arrays of various lengths were engineered for blue energy. • Experiments show that the osmotic power has a local-maximum dependence on channel length, violating the past understanding. • The significant ion concentration polarization in ultrashort nanochannels results in the anomalous decrease in osmotic power.

Unraveling the Anomalous Surface-Charge-Dependent Osmotic Power Using a Single Funnel-Shaped Nanochannel.

Nanofluidic osmotic power, which converts a difference in salinity between brine and fresh water into electricity with nanoscale channels, has received more and more attention in recent years. It is long believed that to gain high-performance osmotic power, highly charged channel materials should be exploited so as to enhance the ion selectivity. In this paper, we report counterintuitive surface-charge-density-dependent osmotic power in a single funnel-shaped nanochannel (FSN), violating the previous viewpoint. For the highly charged nanochannel, the performance of osmotic power decreases with a further increase in its surface charge density. With increasing pH (surface charge density), the FSN enables a local maximum power density as high as ∼3.5 kW/m2 in a 500 mM/1 mM KCl gradient. This observation is strongly supported by our rigorous model where the equilibrium chemical reaction between functional carboxylate ion groups on the channel wall and protons is taken into account. The modeling reveals that for a highly charged nanochannel, a significant increase in the surface charge density amplifies the ion concentration polarization effect, thus weakening the effective salinity ratio across the channel and undermining the osmotic power generated.

Detection of urine protein by a paper-based analytical device enhanced with ion concentration polarization effect

Charged species can be effectively stacked on paper-based analytical device (PAD) taking advantage of ion concentration polarization (ICP) effect. Protein, as ampholyte species, could also be stacked using this effect to improve its detection sensitivity. In this work, a protein detection method was proposed based on online ICP stacking and successfully used for the detection of total protein from urine samples. We showed that proteins from physiological samples can be directly and repeatedly loaded onto the depletion region of the ICP interface established by a piece of cation exchange membrane on a paper fluidic channel, and the protein content can be cumulatively stacked as a narrow band as visually observed by smartphone camera. Colorimetric detection of model protein (phyco) showed that at least 60-fold preconcentration could be achieved with this method. With post-staining of the stacked albumin from artificial urine, a linear response in the diagnostic meaningful range of 50–350 mg/L (R2 = 0.994) was achieved. Total protein from clinical urine samples was detected, and the recovery rate was found in the range of 93–108%, and the RSD was less than 11%. Comparative assays showed no significant difference between the results of this and that of the clinical method. This paper demonstrated the feasibility of online stacking and sensitive detection of proteins from physiological samples using PAD-ICP platform.

Weakly Charged Nanochannels Can Generate High Salinity Gradient Power

Nanofluidic salinity gradient power (NSGP), in which power associated with the salinity gradient can be generated with ion-selective nanochannels, has recently received considerable attention due to its high power density and power generation process without any chemical pollutants. Developing a novel nanopore/nanochannel material to attain high performance of NSGP device is essential to develop this next generation, renewable, clean energy. A general design guideline is that the higher the nanochannel’s surface charge density, the greater the NSGP performance. However, the present study will report the first evidence that an optimal surface charge density of a nanochannel is required to generate higher performance of salinity gradient power, which challenge the past understanding so far. To explain what we found in experiments, a rigorous model with considering the deportation and protonation reactions on the channel wall is developed to investigate the power conversion from a salinity gradient with a pH-regulated nanochannel. The result from our modeling is in good consistent with the experimental observation of the unexpected charge-density dependent NSGP behavior. It is also found that the nanochannel with lower surface charge density can have remarkably higher NSGP performance. The reason to cause the interesting and unexpected phenomenon is due to the charge-density dependent ion concentration polarization effect, which yields the decrease in the effective concentration ratio (actual driving force) across the nanochannel. The results are informative for designing relevant apparatus in renewable energy applications.

Anomalous pH‐Dependent Nanofluidic Salinity Gradient Power

Previous studies on nanofluidic salinity gradient power (NSGP), where energy associated with the salinity gradient can be harvested with ion-selective nanopores, all suggest that nanofluidic devices having higher surface charge density should have higher performance, including osmotic power and conversion efficiency. In this manuscript, this viewpoint is challenged and anomalous counterintuitive pH-dependent NSGP behaviors are reported. For example, with equal pH deviation from its isoelectric point (IEP), the nanopore at pH < IEP is shown to have smaller surface charge density but remarkably higher NSGP performance than that at pH > IEP. Moreover, for sufficiently low pH, the NSGP performance decreases with lowering pH (increasing nanopore charge density). As a result, a maximum osmotic power density as high as 5.85 kW m-2 can be generated along with a conversion efficiency of 26.3% achieved for a single alumina nanopore at pH 3.5 under a 1000-fold concentration ratio. Using the rigorous model with considering the surface equilibrium reactions on the pore wall, it is proved that these counterintuitive surface-charge-dependent NSGP behaviors result from the pH-dependent ion concentration polarization effect, which yields the degradation in effective concentration ratio across the nanopore. These findings provide significant insight for the design of next-generation, high-performance NSGP devices.

Effect of Nanoemitters on Suppressing the Formation of Metal Adduct Ions in Electrospray Ionization Mass Spectrometry.

In the work, we showed that the use of nanoemitters (tip dimension <1 μm, typically ∼100 nm) could dramatically reduce the nonspecific metal adduction to peptide or protein ions as well as improve the matrix tolerance of electrospray ionization mass spectrometry (ESI-MS). The proton-enriched smaller initial droplets are supposed to have played a significant role in suppressing the formation of metal adduct ions in nanoemitters. The proton-enrichment effect in the nanoemitters is related to both the exclusion-enrichment effect (EEE) and the ion concentration polarization effect (ICP effect), which permit the molecular ions to be regulated to protonated ones. Smaller initial charged droplets generated from nanoemitters need less fission steps to release the gas-phase ions; thus, the enrichment effect of salt was not as significant as that of microemitters (tip dimension >1 μm), resulting in the disappearing of salt cluster peaks in high mass-to-charge (m/z) region. The use of nanoemitters demonstrates a novel method for tuning the distribution of the metal-adducted ions to be in a controlled manner. This method is also characterized by ease of use and high efficiency in eliminating the formation of adduct ions, and no pretreatment such as desalting is needed even in the presence of salt at millimole concentration.

Preconcentration of diluted mixed-species samples following separation and collection in a micro-nanofluidic device.

A microfluidic device consisting of a nanoscale Nafion membrane and a polydimethylsiloxane microchannel is proposed for the preconcentration of diluted multi-mixed species samples then following separation and collection. When an electric field is applied across the microchip, an accumulation of the mixed-species sample occurs at the junction between the microchannel and the membrane by means of ion concentration polarization effect. A separation of the sample then takes place due to the difference in the electrophoretic mobilities of the sample components. Finally, the component of interest is guided to a collection reservoir by manipulating the external potential configuration and is trapped in place by means of a magnetically actuated valve. The preconcentration performance of the proposed device is evaluated in both straight and convergent microchannels using a fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) sample. It is shown that a preconcentration factor of 40 times can be achieved using a straight microchannel. By contrast, the preconcentration factor increases to 50 times when using a convergent channel. The practical feasibility of the proposed device is demonstrated by performing the preconcentration, separation, and collection of a mixed FITC-BSA and Tetramethylrhodamine sample.

Preconcentration and Separation of Mixed-Species Samples Near a Nano-Junction in a Convergent Microchannel.

A fluidic microchip incorporating a convergent microchannel and a Nafion-nanoporous membrane is proposed for the preconcentration and separation of multi-species samples on a single platform. In the device, sample preconcentration is achieved by means of the ion concentration polarization effect induced at the micro/nano interface under the application of an external electric field, while species separation is achieved by exploiting the different electrophoretic mobilities of the sample components. The experimental results show that the device is capable of detecting C-reactive protein (CRP) with an initial concentration as low as 9.50 × 10−6 mg/L given a sufficient preconcentration time and driving voltage. In addition, it is shown that a mixed-species sample consisting of three negatively-charged components (bovine serum albumin (BSA), tetramethylrhodamine(TAMRA) isothiocyanate-Dextran and fluorescent polymer beads) can be separated and preconcentrated within 20 min given a driving voltage of 100 V across 1 cm microchannel in length. In general, the present results confirm the feasibility of the device for the immunoassay or detection of various multi-species samples under low concentration in the biochemical and biomedical fields. The novel device can therefore improve the detection limit of traditional medical facilities.

Electro-osmotic pumping and ion-concentration polarization based on conical nanopores.

A numerical investigation is performed into the characteristics of an electro-osmotic pump consisting of a negatively charged conical nanopore. It is shown that the dependence of the flow rectification effect on the bias direction is the reverse of that of the ion current rectification effect. Moreover, the nozzle mode (i.e., the bias is applied from the base side of the nanopore to the tip side) has a higher flow rate compared to the diffuser mode (i.e., the bias is applied from the tip side of the nanopore to the base side). The results showed that the ion-concentration polarization effect occurred inside the conical nanopore, resulting in surface conduction dominating in the ionic current. The ions inside the nanopore are depleted and enriched under the nozzle mode and the diffuser mode, respectively. As a result, the electro-osmotic pump yields a greater pumping pressure, flow rate, and energy conversion efficiency when operating in the nozzle mode. In addition, we also investigated the flow rate rectification behavior for the conical nanopore. The best flow rate rectification factor in this work is 2.06 for an electrolyte concentration of 10(-3) M.

Nanofracture on fused silica microchannel for Donnan exclusion based electrokinetic stacking of biomolecules

Due to Donnan exclusion, charged molecules are prohibited from passing through a channel of electrical double layer scale (nanometers), even though the molecules are smaller than the lowest dimension of the channel. To employ this effect for on-chip pre-concentration, an ion channel of nanometer scale has to be introduced. Here we introduced a simple method of generating a fracture (11-250 nm) directly on the commercially available open tubular fused silica capillary, and a chip comprised of the capillary with the nanofracture was prepared. A ring-disk model of the fracture was derived with which the fracture width can be easily characterized online without any damage to the chip, and the result was validated by a scanning electron microscope (SEM). The fractures can be used directly as a nanofluidic interface exhibiting an obvious ion concentration polarization effect with high current flux. On-chip electrokinetic stacking of SYBR Green I labeled λDNA inside the capillary was successfully demonstrated, and a concentration factor close to the amplification rate of the polymerase chain reaction (PCR) was achieved within 7 min. The chip is inexpensive and easy to prepare in common chemistry and biochemistry laboratories without limitations in expensive microfabrication facilities and sophisticated expertise. More applications of this interface could be found for enhancing the detectability of capillary based microfluidic analytical systems for the analysis of low concentrated charged species.