A Multi-Analyte Blood Test for Cancer Detection and Localization

Cancer, usually referred to uncontrolled growth of abnormal cells, leading to the formation of a tumour. It is strongly believed that the best chance to arrest cancer is its early detection. Nevertheless, it is not possible to detect many tumors until it is grown sufficiently or it spreads across other parts of the body. Numerous efforts are constantly being made by several researchers to develop effective methods for cancer detection. During the uncontrolled growth of cancer cells some of them would die and shed their mutated DNA into the bloodstream. Liquid biopsy test could detect the DNA carrying mutations, which are associated with cancer. However, development of liquid biopsy test that is capable of screening healthy people remains a big challenge. In addition, the inability of the liquid biopsy test to detect the location of the cancer is a major limitation. A team of researchers at Johns Hopkins University School of Medicine, Baltimore led by Nickolas Papadopoulos and Bert Vogelstein have developed a multi-analyte blood test, referred as “CancerSEEK” for early detection of cancer. CancerSEEK is a ‘liquid biopsy test” that examines mutations in cell-free DNA and proteins circulating in the bloodstream (Fig. 1). This research work is funded by National Institute of Health (NIH), USA and the findings of this study is published recently in Science (J. D. Cohen et al., Science 10.1126/science.aar3247 (2018)).

Fig. 1 Schematic of the liquid biopsy test – Tumour cells shed protein and DNA into the blood stream that can be used as biomarkers for early cancer detection

About 1,005 patients diagnosed with ovary, liver, stomach, pancreas, esophagus, colorectum, lung, or breast cancers (Stage I to III) were used to check the ability of CancerSEEK in which none of them have received chemotherapy prior to blood sample collection and none had evident distant metastasis at the time of study. CancerSEEK evaluates the levels of 8 proteins and the presence of mutations in 2,001 genomic positions in 16 different genes, which helps to identify at least eight common types of cancers. Since the test uses a combination of protein biomarkers along with genetic biomarkers, a better sensitivity is achieved without compromising specificity. CancerSEEK is capable of not only identifying the presence of tumours but also localize the organ at which the cancer cells are grown.

CancerSEEK was found to be 98% accurate for tumours in ovary and liver. The median sensitivity of CancerSEEK was estimated to be 73% and 78% for stage II and stage III cancers, respectively. Unfortunately, the success rate of  CancerSEEK for stage I cancer was limited to 43% (Fig. 2(a)). In spite of its low detection ability for stage I cancer, its ability to narrow down the localization of the cancer in 83% of the patients (Fig. 2(b)) makes CancerSEEK as a most reliable method for cancer detection.

Fig. 2 Performance of CancerSEEK: (a) Sensitivity of CancerSEEK by stage; Bars represent the median sensitivity of the eight cancer types and error bars represent standard errors of the median; and (b) Sensitivity of CancerSEEK by tumor type. Error bars represent 95% confidence intervals.

CancerSEEK is expected to be available in the next few years at an estimated cost of less than US$500. Since cancer-related proteins used by Cancer-SEEK could also appear in people with inflammatory diseases such as arthritis, the applicability of this test for such patients is questioned. As an early detection is the key to surgically remove cancer cells before they metastasise, the detection level of 43% for stage I cancers needs to be improved by a large margin.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to: J. D. Cohen et al., Science 10.1126/science.aar3247 (2018)

Fabrication of Copper Nanowires Through Multi-step Anodizing, Electrochemical Barrier Layer Thinning and Electrodeposition

Nanoporous anodic aluminum oxide (AAO) is one of the most commonly employed templates for nanofabrication since the pore diameter, interpore distance, thickness of the oxide, barrier layer and walls can be precisely controlled by a proper choice of anodizing conditions. For the fabrication of metallic nanowires (NW), it is necessary to decrease the barrier layer thickness. In addition, to achieve sufficient electrical contacts at the bottom of the pores, a thin layer of Au has to be deposited either by sputtering or by electrodeposition (ED) using a cyanide bath. Researchers at Military University of Technology, Poland and Delft University of Technology, The Netherlands have proposed a methodology that combines multi-step anodizing, electrochemical barrier layer thinning (BLT) and ED for the fabrication of Cu NW.

Commercial purity aluminum alloy (AA 1050 alloy) was degreased and electropolished (EtOH:HClO₄ 4:1, 0 °C, 20 V, 120 s, Pt grid cathode). A multi-step anodizing protocol was employed to obtain nanoporous anodic aluminium oxide (AAO) templates with a desired nanoporous structure. Mild anodization (MA) in 0.5 M H₂SO₄ with 20 vol.% ethylene glycol (EG) at 0 °C, 20 V and 60 min (Fig. 1, reaction I) was carried out as the first step. The voltage was increased up to 45 V with 0.5 V steps for each 5 s and hard anodizing was performed at 45 V for 1 h (Fig. 1, reaction II). To thin down the bottom of the barrier layer, mild anodizing was carried out in 0.3 M oxalic acid, at 30 °C, 45 V and 30 min (Fig. 1, reaction III). Electrochemical barrier layer thinning (BLT) of multi-step anodized Al alloy was performed in 0.3 M oxalic acid (Fig. 1, reaction IV). A step-wise decrease in voltage and the duration of each voltage step was varied and the suitable conditions for BLT were optimized. For effective opening of the pores at the bottom, the Al alloy at the base was chemically etched using 0.1 M CuCl₂ in HCl. The applicability of the membranes formed using a combination of MA, HA and BLT was ascertained through electrodeposition (ED) of Cu using 0.3 M CuSO₄ and 0.1 M H₃BO₃ at -0.3 V vs. Ag/AgCl for 30 min (Fig. 1, reaction V). The Cu nanowires (NW) were liberated from the AAO template by chemical etching in 5% H₃PO₄ at 30 °C for 45 min (Fig. 1, reaction VI).

Fig. 1 Schematic representation of the various stages involved in the fabrication of Cu nanowires

MA in 0.5 M H₂SO₄ with 20 vol.% EG at 0 °C, 20 V and 60 min enables the formation of a protective oxide layer. The presence of this oxide layer as well as a steady step wise increase in voltage (0.5 V steps for each 5 s) up to 45 V prevents destruction of the Al alloy anode by the high current density avalanche generated during HA at 45 V for 1 h. The MA/HA combination though improved ordering of nanoporous structure in the resultant AAO, the thickness of the barrier layer at the bottom is a critical issue. MA in 0.3 M oxalic acid,    at 30 °C, 45 V and 30 min decreased the thickness of the barrier layer by a reasonable extent, which is suitable for subsequent BLT process. Since the conditions of anodization are mild, the interpore distance and ordering of the pores are maintained. A step-wise decrease in voltage enables BLT of the anodized Al alloy in 0.3 M oxalic acid. Thinning of the barrier layer is effective at selective voltage step and time (Un+1 = 0.75.Un; Δt = 60 s). Under such conditions of BLT, the hexagonal honeycomb-like morphology is maintained at the bottom of the pores (Fig. 2(a)). The applicability of the AAO formed using a combination of MA, HA, MA and BLT is confirmed by ED of Cu NW with a high aspect ratio (Fig. 2(b)). The successful ED of Cu NW confirms efficient BLT and sufficient electrical contact at the electrolyte–aluminum interface that could have facilitated the reduction of Cu²⁺ ions at bottom of the pores.

Fig. 2 FE-SEM micrographs of (a) bottom side of the AAO (after removal of Al alloy using 0.1 M CuCl₂ in HCl) indicating the effectiveness of BLT performed at U_n+1 = 0.75 Un; Δt = 60 s; and (b) ED Cu NW

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: W.J. Stepniowski et al., Journal of Electroanalytical Chemistry, 809 (2018) 59–66.

Atmospheric Pressure Mass Spectrometric Imaging Method for Sub-cellular Imaging of Live Biological Tissue Slices with High Spatial Resolution

Atmospheric pressure matrix-assisted laser desorption ionization (AP-MALDI) method has been used for imaging of biological samples. The sampling protocol of AP-MALDI method involves heating of the samples to improve desorption and ionization of volatile organic compounds, which damages the biological samples and the limits the spatial resolution of the acquired images. Researchers at Department of New Biology, Companion Diagnostics and Medical Technology Research Group, DGIST, Daegu, Republic of Korea, Graduate School of Analytical Science and Technology (GRAST), Chungnam National University, Daejeon, Republic of Korea and KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, Republic of Korea have reported a high spatial resolution atmospheric pressure mass spectrometric imaging method (referred as AP-nanoPALDI MS) for subcellular imaging of living biological tissue slices with a sampling depth down to several tens of μm.

The AP-nanoPALDI MS system used for high-resolution imaging of biological samples consists of a mass analyzer, a sampling stage, a femtosecond (fs) laser oscillator, an AP plasma equipment, and airflow-assisted ion transport equipment. The schematic of the AP-nanoPALDI MS system and imaging device are shown in Fig. 1.

Fig. 1 Schematic of the AP-nanoPALDI mass spectrometric analysis using a combination of fs laser oscillator, atmospheric pressure plasma jet, airflow-assisted ion transfer equipment and imaging systems

The focused fs laser enables desorption of the neutral molecules while the non-thermal AP helium plasma jet device helps to maintain a helium plasma medium above the sample, thus promoting desorption and ionization of the sample. The airflow-assisted ion transfer tube facilitates effective transport of molecules and ions to the mass analyzer. Since biological samples do not effectively absorb the NIR light, rod-shaped gold nanorods modified by polyethylene glycol (mPEG-AuNRs) are embedded inside the tissues, which served as hot spots, rapidly converts the absorbed photon energy into thermal energy, promotes absorption of the NIR light, enhanced the extent of desorption of neutral molecules, increased the intensity of the mass spectra and favours acquisition of high-quality MS images. Analysis of MS images of a mouse hippocampal tissue slice indicates that most of the strong ion signals are under m/z = 500 and the detected ions can be assigned to particular lipids and metabolites such as adenine, cholesterol and monoacylglycerol ions.

Fig. 2 (a) Optical images and (b, c) mass spectrometric images of a mouse hippocampal tissue slice

In spite of the continuous irradiation with a large number of laser shots, the use of mPEG-AuNRs and non-thermal AP plasma jet as post-ionization source prevents thermal damage of the biological specimens. In the absence of AuNRs, mass spectra could not be recorded. Since the images acquired from live tissue slices provide plenty of spatial information for metabolites, the AP-nanoPALDI MS method can also be used for tissue-based drug screening, which is evidenced by a decrease in cholesterol level in hippocampus tissues treated with methyl β-cyclodextrin. The AP nanoPALDI MS imaging technique can be applied for label-free bioimaging applications and tissue-based drug screening.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Jae Young Kim et al., Atmospheric pressure mass spectrometric imaging of live hippocampal tissue slices with subcellular spatial resolution, Nature Communications, 8 (2017) 2113, DOI: 10.1038/s41467-017-02216-6

Detecting cancer from urine sample – Will the development of a nanowire based device open the gates for early diagnoses and timely medical checkups for cancer?

MicroRNAs (miRNAs) encapsulated by extracellular vesicles (EVs) are found in body fluids of patients with malignant diseases as well as with those having a better health. The difference in the EV-encapsulated miRNAs between these two groups of people could be used as a signature to identify various diseases. The miRNAs present in urine could serve as biomarkers for detecting cancer. Unfortunately, the concentration of EVs in urine is extremely low (<0.01 volume %) and hence the most commonly used method of extraction – ultracentrifugation is not capable of extracting nearly 90% of the miRNA species, due to their low abundance. Recently, a nanowire-based device anchored to a microfluidic substrate is fabricated for the efficient collection of EVs and in situ extraction of various miRNAs of different sequences, which is believed to open the gates for urine-based early diagnoses and medical checkups for cancer (Yasui et al., Sci. Adv. 2017;3: e1701133).

Si (100) served as the base substrate (Fig. 1(a)), which was coated with a positive photoresist followed by channel patterning using photo lithography (Fig. 1(b)). A 140 nm thick Cr layer was sputter deposited (Fig. 1(c)), followed by removal of the photoresist layer and thermal oxidation of the Cr layer at 400 °C for 2 h (Fig. 1(d)). The thermally oxidized Cr layer served as the seed layer for subsequent growth of ZnO nanowires using a solution mixture of 15 mM hexamethylenetetramine (HMTA) and 15 mM zinc nitrate hexahydrate at 95 °C for 3 h (Fig. 1(e)). PDMS was poured over the ZnO nanowire grown substrate and cured (Fig. 1(f)). Subsequently, the PDMS was removed from the Si substrate (Fig. 1(g)) and the ZnO nanowires in the PDMS were transferred to another PDMS substrate. The transferred nanowires were uniformly and deeply buried into PDMS while their slightly emerged heads served as growth points for the second nanowire growth (Fig. 1(h)), which was carried out by immersing the PDMS in a solution mixture of 15 mM HMTA and 15 mM zinc nitrate hexahydrate at 95 °C for 3 h (Fig. 1(i)). To enhance contact events between the ZnO nanowires and the EVs as well as to avoid any pressure drop, the ZnO nanowire embedded PDMS substrate was anchored to a herringbone-structured PDMS substrate (Fig. 1(j)).

Fig. 1 (a-j) Various stages involved in the fabrication of nanowire-based device; and (k) schematic of the collection and extraction of EV–encapsulated miRNAs.

The nanowire-based device is capable of detecting around 1000 types of species of miRNAs when compared to the conventional ultracentrifugation method. Using this device, it is possible to extract EV–encapsulated miRNAs within 40 min (collection, 20 min; extraction, 20 min) by introducing just 1 ml of urine sample followed by 1 ml of lysis buffer into the device (Fig. 1(k)). In contrast, the ultracentrifugation method requires 20 ml of urine sample and more than 5 h for collection and extraction. The device enables a four-fold increase in the miRNA expression level with a larger variety of extracted species of miRNAs. The ZnO nanowire-based device is found to be superior to the commonly used ultracentrifugation method in terms of treatment time and RNA extraction efficiency. This attribute is due to its large surface area of ZnO nanowires and their positively charged surface (isoelectric point of 9.50 at pH 6 to 8), which electrostatically attracts the negatively charged EVs in urine sample at pH 6-8. In addition, the mechanical stability of ZnO nanowires, which are firmly anchored to the PDMS substrate helps to retain their strength during buffer flow and enhances the extraction efficiency. The positively charged surface of ZnO nanowires offers benefit in collecting negatively charged objects in urine samples, including exosomes, microvesicles, and EV-free miRNAs.

The ZnO nanowire-based device is believed to help in the early diagnoses and timely medical checkups based on urine miRNA analysis. The method is capable of identifying urinary miRNAs that could potentially serve as biomarkers for detecting bladder, prostate lung, pancreas, and liver cancer.

T.S.N. Sankara Narayanan

Freeze-drying of TiO2 nanorod arrays improves charge transport properties and performance of solar cell

TiO₂ nanorod arrays (NRAs) prepared by hydrothermal treatment, anodic oxidation and sol-gel synthesis have received considerable attention in solar cells, photoelectrochemical water splitting and gas sensors. Conventional air drying of the TiO₂ NRAs could cause aggregation of neighboring nanostructures and distortion of their morphological features, which deleteriously influence their charge transport properties and surface area. In addition, chemically adsorbed halides and alkyl chains might change the surface properties and  influence the interfacial charge transfer process and hence the overall performance of the device. It is important to preserve the vertical alignment of the nanostructures as well as to avoid chemically adsorbed impurities to achieve fast electron transportation, conformal heterojunctions with guest materials and enhanced light scattering. Researchers at School of Advanced Materials and Nanotechnology and Key Lab of Wide Band-Gap Semiconductor Materials and Devices, Xidian University, People’s Republic of China have employed a freeze drying method to dry the TiO₂ nanorod arrays obtained by a two-step hydrothermal process. The freeze-drying post-treatment renders a clean TiO₂ surface and preserves the vertically-aligned nanostructures.

The TiO₂ NRAs were synthesized on FTO (SnO₂:F) substrates by a two-step hydrothermal method. In the first step, a mixture of 1.5 g TiCl₄, 15 ml DI water and 15 ml HCl (36.5 wt%) was subjected to hydrothermal treatment at 150 °C for 6 h. The resultant powder obtained from the first step was treated with a similar solution without TiCl₄ at 150 °C for 3 h in the second step. The air-dried TiO₂ NRAs were directly collected by air-gun blowing, while the freeze-dried samples were obtained after freeze-drying them for 5 h.

The morphological features of air- and freeze-fried TiO₂ NRAs are shown in Fig. 1(a) and Fig. 1(b), respectively. In spite of the method of drying, there is not much difference in the surface morphologies of the TiO₂ NRAs. Nevertheless, the cross-sectional morphology of the air-dried TiO₂ NRAs indicates collapse of the nanorods and destruction of morphology, probably induced by surface tension effect. In contrast, the vertically-aligned nanorod arrays are well preserved for the freeze-dried TiO₂ NRAs since this methodology enables removal of water by sublimation and desorption under vacuum, which avoids solid/liquid interfaces and eliminates the surface tension effect. During freeze-drying, localized energy generated through intermolecular heat transfer enables breaking of the Ti-Cl bonds, desorption and collision of chlorine atoms, resulting in the formation of molecular chlorine. Thus, the freeze-drying post-treatment leads to a “clean” TiO₂ surface with a well-defined morphology of NRAs. The length of the air- and freeze-dried TiO₂ NRAs are ~7.1 μm and ~7.8 μm, respectively. In spite of a slight difference in their colour shade, no apparent difference is observed in their crystallinity.

Fig. 1 Surface and cross-sectional (top insets) SEM images of TiO₂ NRAs obtained by: (a) air-drying; and (b) freeze drying methods (bottom insets: optical images)

The air- and freeze-dried TiO₂ NRAs show a similar UV-visible absorption spectra before loading the dyes. However, after loading the dyes, the freeze-dried TiO₂ NRAs exhibits a decrease in absorption when compared to that of the air-dried ones (Fig. 2(a)). The amount of dye loaded in air-dried TiO₂ NRAs is 69.6 nmol/cm² while for the freeze-dried TiO₂ NRAs it is decreased to 44.3 nmol/cm², following a decrease in its surface area by ~ 36%. However, the perseverance of the ordered nanostructures enables the freeze-dried TiO₂ NRAs to exhibit an enhanced visible-NIR light-scattering performance when compared to that of the air-dried ones (upper inset of Fig. 2(a)). The band gaps of freeze- and air-dried TiO₂ NRAs are 2.91 eV and 2.95 eV, respectively (bottom inset of Fig.2(a)). The flatband potential (E_fb) of the freeze-dried TiO₂ NRA shows a negative shift (~ 0.05 V) when compared to that of the air-dried one. The donor density (N_d) of the freeze-dried TiO₂ NRA is slightly increased when compared to that of the air-dried TiO₂ NRA` from 0.53×10¹⁷/cm³ to 0.6×10¹⁷/cm³. The schematic energy level diagram (Fig.2(b)) indicates that the negative shift in CB and E_F as well as disappearance of the deep acceptor level of freeze-dried TiO₂ NRAs are due to the removal of the adsorbed species that could facilitate charge separation and transport.

Fig. 2 (a) UV-Vis-NIR absorption of the TiO₂ NRAs formed on FTO substrates, (top inset: diffused reflectance spectra; bottom inset: Kubelka-Munk function vs. energy); and (b)  schematic energy level diagrams

The sharp decay in photocurrent with in a second observed for the freeze-dried TiO₂ NRAs when compared to the prolonged duration of decay of photocurrent over 100 s (Fig. 3(a)) observed for the air-dried one suggests the occurrence of an efficient charge extraction across the freeze-dried TiO₂/electrolyte interface. The model dye sensitized solar cells device (top inset of Fig. 3(b)) fabricated using air- and freeze-dried TiO₂ NRAs and the corresponding J-V curves are shown in Fig. 3(b). The PV parameters of the device fabricated using air-dried TiO₂ NRAs are as follows: open-circuit voltage (V_oc) = 0.70 V, short-circuit current density (J_sc) = 6.68 mA/cm², fill factor (FF) = 66%, and power conversion efficiency (PCE) = 3.08%. For the device fabricated using freeze-dried TiO₂ NRAs, the PV parameters are improved as follows: V_oc= 0.68 V, J_sc=8.11 mA/cm², FF= 65%, and PCE=3.60%. The improvement in the PCE of freeze-dried TiO₂ NRAs by ~20% is mainly due to an increase in its J_s.

Fig. 3 (a) Photocurrent decay curves (inset: parallel capacitance vs. applied potential plots); (b) J-V curves (top inset: model of DSSCs using TiO₂ NRAs)

Freeze-drying post-treatment of TiO₂ NRAs preserves the vertically-aligned nanoarchitecture and provides a clean surface, which helps to improve the electronic properties and the performance of the solar cell.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: P. Zhong et al., Freeze-drying as a novel approach to improve charge transport in titanium dioxide nanorod arrays, ChemElectroChem 10.1002/celc.201700572

Detection of Mesopores Using Liquid Photonic Crystals

Determination of pore characteristics of porous materials are very vital for their practical application in drug delivery, oil-water separation, etc. Isothermal adsorption and desorption of N₂, a method commonly employed for the measurement of mesopore volume, pore diameter and surface area, is time-consuming besides involving the use of expensive instrument and consumption of liquid N₂. Researchers at School of Chemistry and Molecular Engineering, Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, China, have developed a novel method to measure pore characteristics of mesoporous materials by mixing them with supersaturated SiO₂ colloidal solution at different temperatures, followed by quick measurement of “absorption induced reflection wavelength changes” of the precipitated liquid photonic crystals (PC). The positive relationship between pore volume (V) and unit mass reflection wavelength change (Δλ/m), and the negative relationship between pore diameter (D) and average absorption temperature (T) are used to determine the pore volume and pore diameter.

Supersaturated solution of SiO₂ particles in various organic solvents spontaneously precipitate to form liquid PCs in which the particle size controls their structural colors and reflection peaks. A supersaturated ethylene glycol-ethanol (EG-EtOH) solution of SiO₂ particles can be used as a “mesopore indicator” and when it is mixed with mesoporous materials, part of the solvent will be irreversibly absorbed into the mesopores, which increases the volume fraction of SiO₂ particles in solution, shrinks the lattice constant of precipitated PCs and monotonically decreases the reflection wavelength.

The “absorption induced reflection blueshift” is schematically illustrated in Fig. 1(a). A supersaturated SiO₂/EG-EtOH solution is placed for several minutes to precipitate red PCs. When pieces of porous silica are carefully spread into the solution (Fig. 1(b)), the red PCs around the porous powders turns green immediately (Fig. 1(c)). The microscopic reflection exhibits a blue shift of 56 nm (from 553 to 609), which indicates shrinkage of lattice constant of PCs that occurs along with solvent transfer to porous substance (Fig. 1(d)).

Fig. 1 (a) Schematic illustration of the measurement of reflection wavelength change of PCs; (b) dried porous silica powders added to the supersaturated SiO₂/EG-EtOH solution; (c) precipitated red photonic crystals turned green around the porous powder; and (d) reflection spectra of colloidal PCs

The pore volume (V) has a positive relationship with the reflection wavelength changes induced by unit mass of porous materials (Δλ/m), which is schematically represented in Fig. 2(a). For mesoporous silica standards, an increase in mesopore mass (m), increases the wavelength change (Δλ) following the “absorption induced reflection blueshift”, which is also reflected by an increase in slope of the “Δλ-m” curve (Figs. 2(b) and 2(c)). A measure of the reflection wavelength change actually is a precise indicator of the amount of absorbed solvent as well as the pore volume (V).

Fig. 2 (a) Schematic representation of the positive relationship between pore volume (V) and the reflection wavelength changes induced by unit mass of porous materials (Δλ/m); (b) increase in wavelength change (Δλ) and an increase in slope of the “Δλ-m” curve with an increase in mesopore mass (m)

The pore diameter (D) has a negative relationship with the average absorption temperature (T) of mesopores, which is schematically represented in Fig. 3(a). When the SiO₂/EG-EtOH solution is mixed with a silica standard with a pore diameter of 1.92 nm, the reflection change of liquid PC (Δλ) is close to zero at low temperature. An increase in temperature from 60 ℃ to 120 ℃ increases the thermal motion and the kinetic energy of the solvent molecules and promotes absorption of solvent into the mesopores, resulting in a gradual increase in Δλ. At temperatures >120 ℃, Δλ reaches a maximum value following a saturation in the absorption of solvent molecules. Simulation of Δλ vs. T curve using Boltzmann function indicates a negative correlation between the average absorption temperature (T) and the pore diameter (D). Accordingly, T is decreased from 93.5℃, to -12.5℃ for mesopore silica standards with D ranging from 1.92 nm to 34.8 nm (Fig. 3(b)-3(g)). It is evident that larger mesopores are easily filled at low temperatures while the smaller pores are filled only at high temperature. The negative correlation between T and D can be used to determine the pore size.

Fig. 3 Schematic representation of negative relationship of pore diameter (D) with average absorption temperature (T) of mesopores SiO₂; (b-g) temperature evolution of reflection change caused by the addition of mesoporous silica standards with different pore volumes

The methodology is validated by testing two well-known mesoporous silica, viz.,  MCM41 and SBA15. The pore volumes and pore diameters estimated by N₂ adsorption-desorption isotherms and BJH pore distributions are 0.898 cm³/g and 2.43 nm for MCM41, and 0.96 cm³/g and 8.04 nm for SBA15. The average absorption temperature measured using the reflection change of the PCs for MCM41 and SBA15 are 88.1°C and 8.5°C, which indicates their pore diameters are 2.33 nm and 10.86 nm. The mass evolution of reflection change shows that the Δλ/m for MCM41 and SBA15 are 2.49 nm/mg and 2.56 nm/mg, which indicate that their pore volumes are about 0.955 cm³/g and 1.00 cm³/g. The pore characteristics of MCM41 and SBA15 measured by N₂ adsorption-desorption and based on the “absorption induced reflection wavelength changes” of PCs are close, suggesting the suitability of the later method as an alternative for the N₂ adsorption-desorption method.

The method based on the measurement of “absorption induced reflection wavelength changes” of the precipitated liquid PC provides a convenient, efficient way to characterize mesoporous materials. This methodology could also be explored for non-silica based porous materials with suitable modification for wettability.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Biting Zhu et al., Liquid Photonic Crystals for Mesopore Detection, Angew. Chem. Int. Ed. 10.1002/anie.201710456

Playing Billiards using Protons Generated by Electrolytic Dissociation of Hydrogen for Intercalation and Ion Substitution

TaS₂ is a well-known 2D layered material and intercalation extends its window of opportunity for a wide variety of applications. Achieving homogeneous intercalation while maintaining high crystallinity using liquid phase method is difficult. Chemical vapor transport (CVT), which involves a direct reaction between the guest vapor and host solid requires high temperature to vaporize the guest materials. Na₃V₂(PO₄)₃ (NVP), due to its high redox potential, excellent thermal stability,  and strong (PO₄)³⁻ polyanion networks, is considered as a promising cathode material for Na ion battery. Substitution of K⁺ ions in place of Na⁺ ions is likely to improve the rate capability and cycling stability. Nevertheless, the large difference in size between K⁺ and Na⁺ ions makes such substitution difficult. Researchers at Hokkaido University, Japan, Xi’an University of Technology, China, Kyushu University, Japan, Tokyo University of Science, Japan and Kyushu Institute of Technology, Japan have demonstrated a new synthesis method, referred as proton-driven ion introduction (PDII), which is based on a solid-state electrochemical reaction for intercalation into TaS₂ and ion substitution of Na₃V₂(PO₄)₃.

Intercalation of alkali metal ions (K⁺, Na⁺, and Li⁺) into TaS₂

A single crystal of plate-shaped TaS_­2 was placed on a carbon cathode. Disk-shaped phosphate glass containing alkali metal ions (K⁺, Na⁺, and Li⁺), referred as the ion-source material was placed above the TaS₂ single crystal. The electrodes were arranged inside a chamber with H₂ atmosphere. The schematic of the experimental set-up used for proton-driven ion introduction (PDII) is shown in Fig. 1. Upon application of a high between the needle-shaped anode and the carbon cathode, electrolytic dissociation of H₂ leads to the formation of protons and electrons. The protons are accelerated into the ion-source material. Penetration of protons pushes the alkali metal ions from the top to the bottom side of the ion source material. In order to maintain charge neutrality, the ion-source material releases the alkali metal ions through its bottom side, which reach the surface of TaS₂. Simultaneously, electrons produced by the corona discharge move from the needle-shaped anode to the TaS₂ single crystal placed over the carbon cathode through the electrode. The electric current flows around the circuit enables electrochemically driven intercalation of TaS_­2, according to the following reaction: TaS₂ + xA⁺ + xe⁻ = A_xTaS₂ (A = alkali metal ion).

Fig. 1 Schematic of the experimental set-up used for proton-driven ion introduction (PDII) – intercalation of alkali metal ions into TaS₂

Intercalation of transition metal ions (Cu⁺ and Ag⁺) into TaS₂

Besides alkali metal ions, transition metal ions such as Cu⁺ and Ag⁺ can also be intercalated  into  TaS₂ by  PDII,  using  CuI and  AgI as  the  respective source materials. Phosphate glasses containing Na, CuI, and TaS₂ were stacked on the carbon  cathode in that  order  from the top. The  schematic of the  experimental set-up used for the intercalation of Cu and Ag into TaS₂ by PDII is shown in Fig. 2(a). The phosphate glass is essential to prevent the formation of poisonous  HI gas. Optical image of the top side of CuI (Fig. 2(b)) indicate a change in colouration of CuI following the replacement of Cu⁺ with Na⁺ ions. The protons drive the release of Na⁺ ions from the phosphate glass, which is then substituted for the Cu⁺ ions on the top surface of CuI. At the bottom side, alkali metal ions and Cu ions are homogeneously intercalated into TaS₂. The three black single crystals represent Cu_xTaS₂ while the red products formed around Cu_xTaS₂ are indeed Cu metal (Fig 2(b)).

The supply rate of Cu⁺ ions from CuI and the diffusion coefficient of Cu⁺ ions in TaS₂ determines the nature of intercalation. When the rate of supply of Cu⁺ ions is  much smaller than the diffusion coefficient of Cu⁺ ions, Cu⁺ ions spread effectively throughout the TaS₂. On the contrary, when the rate of supply of Cu⁺ ions is  much larger than the diffusion coefficient, the surplus Cu⁺ ions are precipitated as Cu metal and the migration of Cu⁺ ions in to TaS₂ is prevented. Hence, to achieve a homogeneous intercalation, the supply rate of Cu⁺ ions should be smaller than its diffusion coefficient. Accordingly, a combination of low treatment temperature and high voltage fails to produce homogeneous intercalation of Cu intoTaS₂ even after 50 h. However, a high treatment temperature and a low voltage allow the formation of a homogeneous single crystal of Cu_2/3TaS₂, within a short treatment time of ~ 12 h (Fig. 2(c)).

Fig. 2 (a) Schematic of the Cu intercalation; (b) Optical images of the top and bottom surfaces of CuI after PDII. (c) Conditions under which homogeneous an partial intercalation of Cu into TaS₂ occurs

Ion substitution (K+ in place of Na+) in Na₃V₂(PO₄)₃

Powdered sample of Na₃V₂(PO₄)₃ (NVP) taken in a Al₂O₃ cylinder was placed on the carbon cathode. Potassium-containing phosphate glass (K⁺ source material) was placed over the Al₂O₃ cylinder (Fig. 3). Upon application of the voltage, the protons replace the K⁺ ions in the phosphate glass and push them into NVP. Unlike the intercalation of ions into TaS₂, during ion substitution, the K⁺ ions will not receive electrons between interfaces since NVP is electrically insulating.  Conversely, the Na⁺ ions discharged from the bottom side of NVP receive electrons and precipitates. Cross-sectional optical images (Fig. 3) indicate a change in colouration of the NVP powder to dark green in the upper region following substitution of K⁺ ions in place of Na⁺ ions. Based on the chemical composition (K_1.69Na_1.20V_2.00P_3.88O_13.83), the upper region is referred as K-NVP which is distinctly different from the NVP in lower region (Na_3.18V_2.00P_3.02O_10.86). Post-annealing at 600 °C improved the crystallization of K-NVP. When compared to conventional solid-state reaction, it would be possible to increase the amount of K substitution by 15 times using PDII.

Fig. 3 Schematic of the ion substitution process – K⁺ ion substitution in the Na⁺ site in Na₃V₂(PO₄)₃ and cross-sectional optical images of Na_3−xK_xV₂(PO₄)₃

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Masaya Fujioka et al., Proton-Driven Intercalation and Ion Substitution Utilizing Solid-State Electrochemical Reaction, J. Am. Chem. Soc., Article in press, DOI: 10.1021/jacs.7b09328

Cyro-electron microscopy reveals the secret of what is limiting the life time of lithium ion batteries

In lithium-ion batteries (LIBs), during charge-discharge cycles, both Li metal and organic electrolyte become unstable. The continuous deposition and stripping of the Li metal results in a large structural change while dendrite growth worsen the situation. Decomposition of the organic electrolyte at the anode leads to the formation of a solid electrolyte interphase (SEI) layer consisting of organic and inorganic components. The changes in the SEI layer as well as the growth directions of the dendrites could alter the efficiency of the system. Since the Li containing electrode material, organic electrolyte and the SEI layer are chemically reactive and sensitive to electron-beam irradiation, it is hard to characterize them using transmission electron microscopy. These attributes poses difficulty in identification of failure mechanism of LIB.

Researchers at the Stanford University, USA, ShanghaiTech University, China, Universität Erlangen–Nürnberg, Germany, National Accelerator Laboratory, USA have developed a cryo-transfer method (Fig. 1) based on cyro-electron microscopy (cyro-EM) and demonstrated that it would be possible to obtain  atomic-resolution images of sensitive battery materials in their native state (Yuzhang Li et al., Science, 358, Issue 6362, 2017, pp. 506-510).

Fig. 1 Preserving and stabilizing Li metal by cryo-transfer method: (a) Li metal dendrites are electrochemically deposited directly onto a Cu TEM grid and then plunged into liquid N₂ after battery disassembly; and (b) The specimen is then placed onto the cryo-TEM holder while still immersed in liquid nitrogen and isolated from the environment by a closed shutter. During insertion into the TEM column, temperature is not increased > –170 °C, and the shutter prevents air exposure to the Li metal.

The cryo-TEM and cyro-SEM images of the electrodeposited Li metal dendrites (Figs. 2(a) and 2(b)) reveal that the dendrite structure is preserved during the cryo-transfer method. Time time-lapse images obtained under constant electron-beam irradiation (~50 e Å^–2 s^–1) in cryogenic conditions (Figs. 2(c), 2(d) and 2(e)) show no signs of damage in the dendrite morphology even after 10 min. The lack of reactivity of the Li metal with liquid N₂, helps the dendrites to retain their electrochemical state so that the relevant structural and chemical information could be obtained. The inferences made in this study reveal that in carbonate-based electrolyte, Li metal dendrites grow as single-crystalline nanowires along three primary growth directions: <111>, <110>, and <211> (Figs. 2(f), 2(g) and 2(h)) with 49% growth along the <111> direction, followed by 32% along <211> and 19% along <110> direction. In spite of growing as single-crystalline nanowires along a linear direction, the Li metal dendrites often change their growth directions.

Fig. 2 (a) Cryo-TEM and (b) Cryo-SEM images of Li metal dendrites depicting that the morphology is preserved by the cryo-transfer method; (c to e) time-lapse images of  Li dendrite; (f to h) growth of Li metal dendrites along: (f) <111>; (g) <110>; and          (h) <211> directions.

The methodology described in this work is likely to provide a complete understanding of the failure mechanisms in high-energy batteries.

T.S.N. Sankara Narayanan

 For more information, the reader may kindly refer: Yuzhang Li et al., Atomic structure of sensitive battery materials and interfaces revealed by cryo–electron microscopy, Science, 358, Issue 6362, pp. 506-510.