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Introduction

1.1 History Background and Brief Overview

In the latter part of the 20th century, nanotechnology experienced a remarkable evolution, transitioning from an abstract concept to a tangible science that revolutionized numerous scientific fields. This period was characterized by the advent of several groundbreaking nanotechnological tools and techniques, which allowed researchers to explore and manipulate matter at the atomic and molecular scales. During this period, nanotechnologies such as atomic force microscopy (AFM), magnetic tweezers (MTs), acoustic tweezers (ATs), optical tweezers (OTs), and others emerged successively, each with its own strengths.

The AFM, also known as the scanning force microscopy (SFM), represents a groundbreaking advancement in nanotechnology and materials science (Figure 1.1). As a high-resolution scanning probe microscope, AFM operates at the nanoscale, offering imaging capabilities that surpass the optical diffraction limit by an astonishing factor of 1000. This remarkable achievement has revolutionized our ability to observe and manipulate matter at the atomic level.

The development of AFM can be traced back to its precursor, the scanning tunneling microscopy (STM), which was conceived by Calvin Quate, Gerd Binnig, and Christoph Gerber at the IBM Zurich Research Laboratory in 1986. Building upon the principles of STM, AFM has evolved into one of the most indispensable tools in nanotechnology, serving critical functions in material manipulation, imaging, and precise measurement at the nanoscale.

The operational principle of AFM centers around a sophisticated detection system consisting of a microcantilever and an extremely sharp probe attached to its end. This configuration allows for the precise "sensing" of surface characteristics through the interaction between the probe and the sample surface. The system's precision is further enhanced by piezoelectric elements, which enable extremely accurate and controlled movements of either the sample or the scanner, typically in the range of nanometers.

Figure 1.1 An AFM generates images by scanning a small cantilever over the surface of a sample.

Source: OverlordQ's / Wikimedia Commons / CC BY-SA 4.0 / https://en.wikipedia.org/wiki/Atomic_force_microscopy#/media/File:Atomic_force_microscope_block_diagram.svg / last accessed by October 03, 2025.

One of the most significant advancements in AFM technology is the development of conductive cantilevers and specialized conductive AFM attachments. These innovations allow researchers to measure the current bias of samples, opening up new possibilities in electrical characterization at the nanoscale. More sophisticated AFM instruments have pushed these capabilities even further, enabling the measurement of current on the probe itself. This capability allows for the assessment of a sample's conductivity and the observation of electron movement on the sample's underside. However, it is important to note that these advanced measurements present considerable technical challenges, and only a select number of research laboratories worldwide have successfully reported consistent and reliable data from such experiments.

The true power of AFM lies in its ability to sense and amplify the interaction forces between the sharp probe on the cantilever and the atoms of the sample being examined. This capability is what enables AFM to achieve its extraordinary atomic-level resolution. By precisely measuring these atomic-scale interactions, AFM provides researchers with unprecedented insights into the structure and properties of materials at the most fundamental level.

The applications of AFM span across numerous scientific disciplines, including materials science, nanotechnology, biology, and surface chemistry. Its ability to operate in various environments (vacuum, liquid, or air) and modes (contact, noncontact, or tapping) makes it an exceptionally versatile tool for nanoscale research and development. As the AFM technology continues to evolve, it promises to unlock even more possibilities in our understanding and manipulation of matter at the atomic scale, driving innovation in fields ranging from semiconductor technology to biomedical research [1].

Magnetic tweezers (MT) have become a significant and adaptable tool in biophysics and soft matter studies, facilitating the precise control and analysis of single biomolecules or polymers (Figure 1.2). These advanced instruments are engineered to apply specific forces and torques at the molecular level, offering scientists an unparalleled understanding of the mechanical attributes and behaviors of biological macromolecules.

Figure 1.2 Typical configuration for magnetic tweezers.

Source: Daniel Förster / Wikimedia Commons / CC BY-SA 4.0 / https://en.wikipedia.org/wiki/Magnetic_tweezers#/media/File:Magnetic_tweezers.svg / last accessed by October 03, 2025.

The principal function of MTs is their capacity to assess foundational mechanical properties, such as the tensile strength of molecules or the forces involved in molecular interactions. This ability has rendered them, especially useful in single-molecule experiments, where they are predominantly used to examine the mechanical characteristics of biological macromolecules like DNA and proteins. In addition to these uses, MTs have proved beneficial across a variety of research domains, including the study of the rheology of soft matter systems and the exploration of force-dependent processes in living cells. The usual range of forces for these experiments extends from piconewtons to nanonewtons, making them perfectly suited for examining the delicate mechanical interactions that underpin biological functions.

One of the key advantages of MTs is their relatively simple and robust architecture, which has contributed to their widespread adoption as a biophysical tool. The experimental setup typically involves tethering the molecule of interest to a magnetic microparticle, which serves as the handle for manipulation. The MT system is equipped with precisely controlled magnets that generate the necessary magnetic fields to manipulate these particles in three-dimensional space. The position and movement of the magnetic particles are then monitored with high precision using advanced video microscopy techniques, allowing for the real-time tracking and analysis of molecular behavior.

The versatility of MTs extends their ability to perform both force spectroscopy and torque measurements, making them particularly valuable for studying the mechanical properties of DNA and proteins. For instance, they have been instrumental in elucidating the mechanisms of DNA supercoiling, protein folding, and the behavior of molecular motors. In soft matter research, MTs have been employed to investigate the viscoelastic properties of complex fluids and the mechanical response of biological membranes.

Recent advancements in the MT technology have further enhanced their capabilities, including improvements in force resolution, the development of multiplexed systems for parallel measurements, and the integration with other single-molecule techniques. These developments have opened up new possibilities for studying complex biological processes and material properties at the molecular level [2].

Acoustic tweezers (AT) represent a cutting-edge technology that harnesses the power of sound waves to precisely manipulate the position and movement of microscopic and sub-millimeter objects (Figure 1.3). This innovative approach to particle manipulation has opened up new possibilities in various scientific fields, particularly in biomedical research and nanotechnology. At its core, ATs carefully utilize controlled sound waves to create acoustic pressure fields that can influence the position of small objects. While the term "AT" strictly refers to a single-beam configuration, the broader concept encompasses two primary configurations: single-beam and standing-wave setups. The technology operates by manipulating the position of acoustic pressure nodes, which serve as stable points that attract and hold objects within a standing acoustic field. For effective manipulation, the target object must be significantly smaller than the wavelength of the sound used, typically ranging from microscopic particles to individual cells.

Figure 1.3 Acoustic radiation force on a small particle.

Source: Rsla1 / Wikimedia Commons / CC BY-SA 4.0 / https://en.wikipedia.org/wiki/Acoustic_tweezers#/media/File:Figure1_zhangming.png / last accessed by October 03, 2025.

One of the most significant advantages of ATs is their noninvasive nature and safety for biological applications. Unlike other manipulation techniques that might involve potentially harmful forces or radiation, acoustic waves have been proven to be gentle and safe for biological objects. This characteristic makes ATs particularly valuable in biomedical research, where maintaining cell viability and integrity is crucial. Recent advancements in the AT technology have expanded its applications across various domains, including flow cytometry, where they have been integrated into systems to enable more precise cell sorting and analysis without the need for physical contact or labels. They have also proven effective in separating different cell types based on their physical properties, offering a label-free alternative to traditional...