Microsphere Probe Research Articles

Detection of picometer scale vibration based on the microsphere near-field probe

We introduce an innovative vibration detection method utilizing a microsphere near-field probe, affixed to an optical fiber probe. Notably, when the microsphere probe is positioned at a distance of approximately 100 nm from the vibrating objects, it exhibits a substantial increase in detection sensitivity, enabling the detection vibration as small as 5 pm. This enhancement arises from the strong near-field interaction between the sample and probe, where the evanescent wave dominates. This effect is confirmed for the first-time through both theoretical analysis and experimental validation. This innovative probe has great potential for applications in cellular acoustics, vibrational detection of biomolecules and their complexes, in situ measurements, and imaging of microstructures and nanostructures.

A new mechanical engineering strategy based on microsphere probe and its application in transition metal dichalcogenides

Mechanical engineering of 2D materials allows continuous and reversible modulation of their electronic and photonic properties. Although photoluminescence (PL) measurement is an effective way to monitor the effects of mechanical forces on 2D semiconductors, there is currently a lack of techniques to enhance PL signals during stress application. This study presents an innovative mechanical engineering approach that integrates a dielectric microsphere as an atomic force microscopy (AFM) probe into a Raman-AFM system. Force–distance curve tests and COMSOL simulations were performed to analyze and estimate the compressive stress exerted by the microsphere. Importantly, the PL signals of transition metal dichalcogenides subjected to microsphere probe's force were enhanced and reveal distinct mechanical responses depending on the substrate rigidity: compressive pressures for rigid substrates and tensile strains for flexible ones. Notably, this strategy not only amplifies spectral signals in real time but also achieves fine stress modulation in the precise targeted material region, demonstrating its superiority in sensitive mechanical engineering applications. Our work offers a new avenue for the deliberate design of mechanical strains in 2D materials, which is crucial for optimizing the performance of related devices.

Degradation induced superlubricity on the rough surface of black phosphorus composite

A rough surface composed of asperities was considered as the obstacle for achieving superlubricity in traditional friction theory. Here, microscale superlubricity with the friction coefficient of 0.0086 was observed on a relatively rougher soft surface of black phosphorus (BP) composite due to the degradation of BP with oxygen and moisture via the atomic force microscopy (AFM) experiment equipped with diamond-like carbon (DLC) microsphere probe. Comparative analyses corroborated that the produced phosphorus oxides (oBP) with abundant P=O and P−OH bonds and the local graphitized amorphous carbon film at the sliding interface decreased the possibility of direct contact between original rough peaks and provided an ultra-low shear strength for achieving superlubricity. Furthermore, a certain nanoscale roughness structure on the composite surface facilitated a more reliable superlubricity state, which supplied the larger contact area and locking capillary force to hold and retain the lubricating liquid film formed by the water absorption of oBP. These findings provided fundamental insights into the nature of tribological mechanisms and suggested a new method to achieve ultra-low friction of polymer composite materials in engineering.

A portable deep-learning-assisted digital single-particle counting biosensing platform for amplification-free nucleic acid detection using a lens-free holography microscope

The digital single-particle assay has emerged as a highly promising technology in various detection applications, such as food safety inspection, environmental monitoring, and in vitro diagnosis. Conventional digital assays rely on pre-amplification and expensive equipment, which limits their practical applications to point-of-care testing. Herein, we report a deep-learning-assisted digital single-particle counting biosensing platform for nucleic acid detection without pre-amplification using a portable and low-cost lens-free holography microscope. This device can perceive the number change of signal probes and capture microsphere probe holograms, which is ultra-lightweight (∼ 318 g), and has a low cost (∼ $70) and an ultrawide field of view of 24.396 mm2. The improved YOLOv7-based deep learning algorithm is trained to detect small objects (∼ 10 μm) in high-resolution images with high throughput. As a proof of concept, our strategy has successfully distinguished viable and nonviable Salmonella typhimurium quantitatively with high sensitivity (72 CFU/mL) without pre-amplification using phage-mediated DNA extraction and has been verified in various real samples. It has also been successfully applied to detecting non-nucleic acid targets in real samples, including procalcitonin and chloramphenicol. As a versatile and multi-functional platform, this platform exhibits excellent potential for point-of-care multi-type target detection in resource-limited settings.

Preparation and mechanism study of hydrogen bond induced enhanced composited gelatin microsphere probe

Fluorescent probes offer rapid and efficient detection of metal ions. However, their properties, including high biotoxicity and low detection limits, often limit their utility in biological systems. In this study, we used a microfluidic approach to fabricate photocrosslinked gelatin microspheres with a micropore, providing a straightforward method for loading fluorescent probes into these microspheres based on the adsorption effect and hydrogen bonding interaction. The gelatin microsphere loaded probes, GelMA/TPA-DAP and GelMA/TPA-ISO-HNO were designed and obtained. The results show that these probes exhibit obviously low biotoxicity compared to the original molecular probes TPA-DAP and TPA-ISO-HNO. Simultaneously, it is found that GelMA/TPA-DAP and GelMA/TPA-ISO-HNO have better detection sensitivity, the detection limits are 35.4 nM for Cu2+, 16.5 nM for Co2+ and 20.5 nM for Ni2+ for GelMA/TPA-DAP probe. Compared to the original TPA-DAP they are improved by 37.2 %, 26.3 % and 22.6 % respectively. The corresponding coordination constants were 10.8 × 105, 4.11×105 and 6.04×105, which is larger than homologous TPA-DAP. Similar results were also verified in the GelMA/TPA-ISO-HNO probe. The mechanism was investigated in detail by theoretical simulations and advanced spectral analysis. The density functional theory (DFT) simulations show that the probes are anchored inside the microspheres and the molecular structure is modified due to the hydrogen bonding interaction between the microsphere and the molecular probe, which makes GelMA/TPA-DAP exhibit stronger coordination capacity with metal ions than homologous TPA-DAP. In addition, the adsorption effect also provided some synergistic enhancement contribution. Meanwhile, cellular experiments have also shown that the composite microspheres can improve the biocompatibility of the probe and will provide a wider range of applications towards bioassay. This simple and effective method will provide a convenient way to improve the performance of fluorescent probes and their biological applications.

Microsphere probe: combining microsphere-assisted microscopy with AFM.

In recent years, microsphere-assisted microscopy (MAM) and atomic force microscope (AFM) have been rapidly developed to meet the measurement needs of microstructures. However, the positioning of microspheres, the inability of AFM to touch the underlying sample through the transparent insulating layer, and the challenge of AFM fast positioning limit their use in practical measurements. In this paper, we propose a method that combines MAM with AFM by adhering the microsphere to the cantilever. This method allows MAM and AFM to work in parallel, and their imaging positions can correspond with each other. We use this method to measure memory devices, and the results show that MAM and AFM yield complementary advantages. This approach provides a new tool for analyzing complex structures in devices and has potential for wide application.

Near-Field Picometer Vibration Detection Based on Dielectric Microsphere Probe

We propose a near-field picometre vibration detection method based on a dielectric microsphere probe. Using an optical fiber combined with a dielectric microsphere to form a near-field probe. When parallel light is injected into the dielectric microsphere, interaction occurs between the surface of the microsphere and the focused beam in the near-field, at the front end of the dielectric microsphere will generate a highly localized electromagnetic beam, i.e., a photonic nanojet(PNJ). The nanojet of the probe in the near-field can enhance the scattered light of the object under the test surface. We used a differential structure that was formed by two optical fiber probes to detect the picometre vibration in the near-field. From the experimental results, we know that the method can detect the picometre vibration signal, while the minimum vibration signal of 20 pm can be detected. The method has the potential to apply in cellular diagnostics, single-molecule detection, and in situ measurements.

Study on an all-fiber optical coherence tomography (OCT) probe

An all-fiber probe is proposed to meet the requirements of small size and strong light-collection capability for optical coherence tomography (OCT) probes for in vivo human detection. It consists of a single-mode fiber, a coreless fiber and a gradient refractive index (GRIN) fiber. The coreless fiber is fused into a spherical shape to focus the beam and the GRIN fiber is added to the microsphere probe to optimise the resolution. The optical transmission model of the probe is developed, and then the structural design of the probe is modelled and simulated to determine the optimal parametric structure of the probe. Moreover, apply the probe to the OCT system for the imaging experiments to obtain grey-scale interior images of three- layer of slides, which can verify the feasibility of the probe. The diameter of the probe is 300 μm, which meets the size requirements for in vivo human detection and provides a highly compact, high-resolution option for the field of in vivo detection.

Ultra-low friction and stiffness dependence of interlayer friction in graphite flakes under various rotation angles

The 60-degree rotational symmetry of graphene layers leads to remarkable physical and mechanical phenomena. One corresponds to ultralow sliding friction and friction coefficient between incommensurate graphene layers. A pick-up method was proposed to pick graphite flake to microsphere probe and achieved graphite and graphite tribo-pair. Ultralow friction was attained for various rotation angles, and the friction coefficient was kept at the range of 10−3. Density functional theory (DFT) calculation was employed to reveal the angular dependence of friction, and the calculations are partially consistent with the experimental results. The difference of angle-dependence ultralow friction between experimental results and DFT calculations was observed, which was greatly influenced by the stiffness of the system. Friction hysteresis, abnormal shape of pull-off curves and long jumps in stick-slip were further demonstrated the stiffness of the system had changed during the friction measurements. The contact geometry in our experiment was a finite, symmetric, nanometer-sized (less than 1 nm), flexible graphite flake and a rigid graphite surface. The tribological system offers an experimental platform to study interlayer friction and contact geometry.

The Transition of Friction Coefficient on Graphene by the Microsphere Probe

The Transition of Friction Coefficient on Graphene by the Microsphere Probe

Microscopic Insights into the Effects of Anti-Agglomerant Surfactants on Surface Characteristics of Tetrahydrofuran Hydrate

The formation of clathrate hydrates in pipelines is potentially threatening to exploration and gas transportation in the petroleum industry. To reduce such risks, various surfactants have been explored as anti-agglomerants to prevent the aggregation of hydrate particles. However, the anti-agglomeration mechanisms are not yet fully understood. In this study, modified atomic force microscopy is first developed to investigate the effects of two surfactants, namely, 1-naphthaleneacetic acid and dodecylbenzenesulfonic acid, on the surface of a tetrahydrofuran (THF) hydrate. The surfactants have remarkable effects in terms of changing the grain size and decreasing the grain boundary depth and surface roughness of the THF hydrate. In addition, the surfactants reduce the quasi-liquid layer (QLL) thickness of the THF hydrate and the adhesion forces between the hydrate and the microsphere probe. This phenomenon may be caused by the changes induced by surfactant molecules on the water–guest molecule structure near the gas/water interface. Thus, hydrate growth is enhanced in the QLL. The adhesion forces decrease linearly with QLL thickness after adding the surfactants. These findings indicate that surfactants reduce the adhesion forces by reducing the QLL thickness at higher temperatures. The effects of surfactants on the QLL thickness and surface morphology of the hydrate are investigated, providing critical information on the cohesive behaviors among hydrate particles or between hydrate particles with other materials. Our work provides new insights into the underlying mechanism of how surfactants prevent hydrate aggregation, which is crucial in hydrate-related safety management.

Flexible Tuning of Friction on Atomically Thin Graphene.

The tuning of flexible microscale friction is desirable for the reliability of wearable electronic devices, tactile sensors, and flexible gears. Here, the tuning of friction of atomically thin graphene on a flexible polydimethylsiloxane (PDMS) substrate was obtained with the elastic modulus using a 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) self-assembly monolayers (SAMs)-modified microsphere probe with the diameter of 5 μm at the microscale. The friction can be tuned at a large scale with the difference in the elastic modulus of PDMS and thickness of graphene. The hydrophobic property of the FDTS SAMs-modified probe decreased friction by reducing interfacial adhesion and preventing the effect of capillary interaction; thus, the friction decreased with the increase in the elastic modulus of the PDMS substrate due to decreasing indentation depth and thus the interfacial contact area; and also, the enhanced out-of-plane stiffness effectively decreased the interfacial contact quality with the increase of the thickness of graphene. The flexible tuning of friction on graphene was further verified by the theoretical calculation from the aspects of the friction arising from the normal and lateral deformation around the contacting area. This work is meaningful for promoting the design and reliability of flexible micro-devices.

Study on the microcosmic superlubricity mechanism of PVPA affected by metal cations

Hydrophilic polymer coatings on artificial implants generate excellent tribological properties. The friction properties of polymer coatings are affected by salt ion factors. Herein, the atomic force microscopy (AFM) was used to show that the superlubricity was achieved between poly(vinylphosphonic acid) (PVPA)-modified Ti6Al4V and polystyrene (PS) microsphere probe lubricated with monovalent salt solutions (LiCl, NaCl, KCl, and CsCl). Considering that adhesion is an important cause of friction changes, the AFM was further utilized to obtain adhesion between friction pairs in different salt solutions. The results indicated that the larger the cation radius in the lubricant, the smaller the adhesion, and the lower the friction coefficient of the PVPA coating. The electrostatic interaction between the PVPA and one-valence cations in lubricants was analyzed by the molecular dynamics (MD) simulation as it was found to be the main influencing factor of the adhesion. Combined analysis results of friction and adhesion indicated that by adjusting the size of cation radius in lubricant, the adhesion between the tribo-pairs can be changed, and eventually the magnitude of friction can be affected. This study opens up a new avenue for analyzing the friction characteristics of hydrophilic polymer coatings from the perspective of intermolecular forces.

A dual-functional magnetic microsphere for ICP-MS quantification and fluorescence imaging of matrix metalloproteinase 2 in cell secretion

Matrix metalloproteinase 2 (MMP2) plays an important role in tumor growth, invasion and metastasis. In this work, a dual-functional magnetic microsphere probe was designed for ICP-MS quantification and fluorescence imaging of MMP2 in cell secretion. In the designed probe, a NH2-peptide (-FAM)-biotin was used as a bridge for the combination of carboxylated magnetic beads (MBs-COOH) and streptavidin functionalized gold nanoparticle (Au NP-SA). Initially, the fluorescence of FAM was quenched by Au NP. Since the NH2-peptide (-FAM)-biotin had a MMP2-specifically recognized sequence, the peptide was specifically cleaved in the presence of MMP2, thus releasing Au NP for the ICP-MS quantification of MMP2 and turning on the fluorescence of FAM for the fluorescence imaging of MMP2. Under the optimal experimental conditions, a linear range of 0.05–50 ng mL−1 and a limit of detection of 0.02 ng mL−1 were obtained for MMP2. The relative standard deviation (n = 7, c = 0.1 ng mL−1) of the proposed method was 5.4%. With good sensitivity and good accuracy, the proposed method realized the quantification and imaging of MMP2 in A549 cell secretion. The proposed method was applied to monitor the expression of MMP2 in the A549 cell secretion under the stimulation of Cd2+, providing a new detection strategy in the study of MMP2-related life process.

Highly Expressed Amyloid Beta-42 Of Aqueous Humor In Patients With Neovascular Macular Degeneration

ABSTRACT Background: Age-related macular degeneration (AMD) is a type of macular degeneration disease, and amyloid beta (aβ) is the main component of vitreous warts in AMD patients. Neovascular AMD (nAMD) is the most serious type of AMD, but its pathogenesis remains unclear. The aim of this study was to detect the expression of aβ42 in the aqueous humor of nAMD patients and to evaluate whether aβ42 expression of aqueous humor is correlated with cognitive function in these patients. Methods: A total of 70 patients were enrolled in this study, including 50 nAMD patients (nAMD group) and 20 patients with cataract (control group). The cognitive function of the patients was assessed using the Mini-Mental State Examination and Montreal Cognitive Assessment Scale, and based on their scores, 50 patients with nAMD were divided into two subgroups: the p-nAMD group (18 nAMD patients with normal cognition) and the ci-nAMD group (32 nAMD patients with cognitive impairment). An immunofluorescence microsphere probe technique was used to detect the aβ42 expression of aqueous humor in all patients. Pearson correlation analysis was used. Results: The aβ42 expression of aqueous humor was significantly higher in the nAMD group (124.56 ± 41.93 pg/mL) as compared with the control group (82.94 ± 33.75 pg/mL; P < .01). There was no significant difference in aβ42 expression of aqueous humor between the p-nAMD group (136.42 ± 51.68 pg/mL) and ci-nAMD group (117.90 ± 34.46 pg/mL; P = .14). Conclusion: In nAMD patients, aβ42 was highly expressed in the aqueous humor but was not correlated with cognitive function.

Synthesis and deposition of Silver nanostructures on the silica microsphere by laser-assisted photochemical method for SERS applications

The homogeneous distribution of nano-metallic structures on the surface-enhanced Raman (SERS) substrates plays an important factor for high-sensitive Raman scattering measurement. In this paper, we present a low-cost laser-assisted photochemical method for making a SERS probe based on silver nanostructures, which are one-timely synthesized nano-silver structures, homogeneously deposited on silica microsphere surfaces. Achieved SERS-activity substrates with a homogeneous distribution of Ag-nanostructures are verified by a mapping technique on the surface of Ag-coated microsphere for the detection of low concentration of Rhodamine 6G in aqueous solutions in a range of 10-4-10-9M. The obtained results show that a SERS microsphere probe has a good repetition of SERS-activity in any space of sensing area, and large potential for application in a biochemical sensing technique. Full Text: PDF ReferencesY. Chen et al., "Interfacial reactions in lithium batteries", J. Phys. D: Appl. Phys. 50, 02510 (2017). CrossRef T.B. Pham, H. Bui, H.T. Le, V.H. Pham, "Characteristics of the Fiber Laser Sensor System Based on Etched-Bragg Grating Sensing Probe for Determination of the Low Nitrate Concentration in Water", Sensors 17, 0007 (2017). CrossRef X. Wang, O.S. Wolfbeis, "Fiber-Optic Chemical Sensors and Biosensors (2013–2015)", Anal. Chem. 88, 203 (2016). CrossRef R. Wang, K. Kim, N. Choi, X. Wang, J. Lee, J.H. Joen, G. Rhie, J. Choo, "Highly sensitive detection of high-risk bacterial pathogens using SERS-based lateral flow assay strips", Sens. Actuators B-Chem. 270, 72 (2018). CrossRef H. Zhang et al., "Determination of Pesticides by Surface-Enhanced Raman Spectroscopy on Gold-Nanoparticle-Modified Polymethacrylate", Anal. Let. 49, 2268 (2016). CrossRef L. Chen, H. Yan, X. Xue, D. Jiang, Y. Cai, D. Liang, Y.M. Jung, X.X. Han, B. Zhao, "Surface-Enhanced Raman Scattering (SERS) Active Gold Nanoparticles Decorated on a Porous Polymer Filter", Appl. Spectrosc. 71, 1543 (2017). CrossRef A. Matikainen, T. Nuutinen, P. Vahimaa, S. Honkanen, "A solution to the fabrication and tarnishing problems of surface-enhanced Raman spectroscopy (SERS) fiber probes", Sci. Rep. 5, 8320 (2015). CrossRef J. Zhang, S. Chen, T. Gong, X. Zhang, Y. Zhu, "Tapered Fiber Probe Modified by Ag Nanoparticles for SERS Detection", Plasm. 11, 743 (2016). CrossRef W. Xu et al., "A Dual-Butterfly Structure Gyroscope", Sensors 17, 467 (2017). CrossRef K. Setoura, S. Ito, M. Yamada, H. Yamauchi, H. Miyasaka, "Fabrication of silver nanoparticles from silver salt aqueous solution at water-glass interface by visible CW laser irradiation without reducing reagents", J. Photochem. Photobio. A: Chem. 344, 168 (2017). CrossRef K. Liu, Y. Bai, L. Zhang, Z. Yang, Q. Fan, H. Zheng, Y. Yin, C. Gao, "Porous Au–Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis", Nano Lett. 16, 3675 (2016). CrossRef Z. Huang, X. Lei, Y. Liu, Z. Wang, X. Wang, Z. Wang, Q. Mao, G. Meng, "Tapered Optical Fiber Probe Assembled with Plasmonic Nanostructures for Surface-Enhanced Raman Scattering Application", ACS Appl. Mater. Interfaces 7, 17247 (2015). CrossRef

Fluorescent microsphere probe for rapid qualitative and quantitative detection of trypsin activity

Current technologies still face a big challenge to achieve simultaneous rapid qualitative and quantitative detection of trypsin. In our present study, we developed a simple and effective strategy to sensitively, qualitatively and quantitatively analyze the activity of trypsin using a fluorescent polystyrene (PS) microsphere probe. PS spheres were first functionalized by the surface coating of polyethylene glycol (PEG), which could significantly decrease the possibility of nonspecific physical adsorption of the fluorescein isothiocyanate isomer-modified peptide (peptide-FITC). Then, the obtained PS-PEG spheres were chemically interacted with peptide-FITC, which were then employed to monitor the real-time activity of trypsin. The peptide used in our work contained rich lysine and arginine residues, which were the recognition sites of trypsin. When trypsin interacted with the PS-FITC-peptide microspheres, the peptide-FITC rapidly decomposed into free small fragments in solution, resulting in a gradual decrease in the fluorescence of the PS spheres. By taking advantage of the fluorescence changes using confocal microscopy imaging and fluorescence spectrum intensity, it is easy to achieve the qualitative and quantitative detection of trypsin, with a highly sensitive detection limit as low as 0.5 ng mL−1 and high selectivity. Thus, the designed fluorescent PS microsphere probe would be very promising in various applications such as food safety inspection, personal healthcare and on-site environmental monitoring.

Robust microscale superlubricity under high contact pressure enabled by graphene-coated microsphere

Superlubricity of graphite and graphene has aroused increasing interest in recent years. Yet how to obtain a long-lasting superlubricity between graphene layers, under high applied normal load in ambient atmosphere still remains a challenge but is highly desirable. Here, we report a direct measurement of sliding friction between graphene and graphene, and graphene and hexagonal boron nitride (h-BN) under high contact pressures by employing graphene-coated microsphere (GMS) probe prepared by metal-catalyst-free chemical vapour deposition. The exceptionally low and robust friction coefficient of 0.003 is accomplished under local asperity contact pressure up to 1 GPa, at arbitrary relative surface rotation angles, which is insensitive to relative humidity up to 51% RH. This ultralow friction is attributed to the sustainable overall incommensurability due to the multi-asperity contact covered with randomly oriented graphene nanograins. This realization of microscale superlubricity can be extended to the sliding between a variety of two-dimensional (2D) layers.

Fabrication of Porcine DNA Biosensor Based on Ruthenium Bipyridine Complex

Electrochemical DNA biosensor for detection of porcine oligonucleotides based on ruthenium (II) as label redox complex has been developed. The system of biosensor is based on the modified screen printed carbon electrode (SPE) with goldnanoparticles (AuNPs) as transducer immobilized with poly(n-butylacrylate-N-acryloxysuccinimide) microsphere and porcine DNA probe sequences was attached onto it via the covalent bond. The ruthenium(II) complex, [Ru(bpy)2PIP]2+ PIP = 2-phenylimidazo[4,5-f[1,10-phenanthroline] intercalator has been been used to determine prcine DNA. The biosensor was measured and optimized by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The interaction of [Ru(bpy)2PIP]2+ with various DNA with different sequences, i) single stranded probe DNA, ii) after hybridization with its complementary DNA and iii) after hybridization with mismatch complementary DNA were studied. The results indicated that the interaction of [Ru(bpy)2PIP]2+ with hybridized complementary DNA gave the highest response. Thus, development of porcine DNA biosensor and influences of many factors such as [Ru(bpy)2PIP]2+ complex and DNA probe concentration, DNA probe immobilization and hybridization time, pH, ionic strength, buffer concentration and temperatures were also studied to evaluate the performance of biosensor. The concentration of [Ru(bpy)2PIP]2+ complex and DNA probe were found to be optimal at 50 µM and 2 µM, respectively. The optimal time for DNA probe immobilization and hybridization were 7 hours and 60 minutes, accordingly. The optimal condition of pH, ionic strength, buffer concentration and temperatures were at pH 7.0, 1.0 M NaCl, 0.05 M of Na-phosphate buffer and 25 °C, respectively. The linear range of different concentration complementary DNA was within the range between 1.0 x 10-13 M to 1.0 x 10-8 M. This study was first reported the used of [Ru(bpy)2PIP]2+ for detection of porcine oligonucleotides.

Interactions between Solid Surfaces Mediated by Polyethylene Oxide Polymers: Effect of Polymer Concentration

Using total internal reflection microscopy (TIRM), we have systematically measured the interactions between a microsphere and a flat hydrophilic surface in the presence of polyethylene oxide (PEO) polymer solution. Our results reveal that PEO significantly mediates the interaction forces between the two surfaces. At low polymer concentration, the interactions between two surfaces in the presence of PEO are mainly dominated by repulsive forces, originating from diffuse layer overlap. At intermediate polymer concentration, a long-range and weak attraction sets in. This force is likely attributed to the depletion attraction due to the presence of free PEO chains in bulk solution; however, a simple hard-sphere AO model fails to precisely describe the attraction. At high polymer concentration where PEO chains overlap, the attraction disappears, and levitation of the microsphere probe is detected. We argue that at this overlapping region, the correlation length of PEO chains is much smaller than the size of single PEO molecule, leading to weakening and disappearing of the depletion attraction. Finally, at very high concentration, oscillatory structural force is obviously found, indicating the significant structural ordering of the PEO chains under confinement.