How Scientists Are Weighing and Analyzing Single Nanoparticles
Exploring the breakthrough technology that combines optical trapping with laser-induced plasma imaging to analyze attogram masses
Imagine trying to identify and weigh a single dust particle floating in a large auditorium—while the lights are off and the air is constantly moving. Now, scale down that challenge by a factor of a billion, and you'll begin to appreciate the extraordinary task facing scientists who study nanoparticles. These vanishingly small specks of matter, measuring between 1 and 100 nanometers in size, have become the building blocks of modern technology, appearing in everything from medical treatments to electronic devices. Yet, their minute size—often consisting of mere attograms (10⁻¹⁸ grams) of material—has made them nearly impossible to isolate and analyze individually.
Record-breaking sensitivity achieved
Analysis of individual nanoparticles
Simultaneous detection capability
Until recently, scientists could only study nanoparticles in bulk, like trying to understand the properties of snowflakes by examining a snowball. Traditional analysis methods require relatively large sample amounts, masking the unique characteristics of individual particles. This limitation became increasingly problematic as researchers discovered that minor variations between individual nanoparticles can dramatically alter their behavior and effectiveness in applications ranging from drug delivery to environmental monitoring.
This article explores a remarkable scientific breakthrough that combines the precision of optical trapping (using laser beams as "tractor beams") with the analytical power of laser-induced breakdown spectroscopy (LIBS). This innovative approach allows researchers not only to capture individual nanoparticles in mid-air but to determine their exact composition and mass with unprecedented sensitivity—down to the attogram level. The implications of this technology span across medicine, materials science, and environmental protection, opening new frontiers in our understanding of the nanoscale world.
Using focused laser beams as "optical tweezers" to capture and manipulate individual nanoparticles in mid-air without physical contact.
Developed by Arthur Ashkin, who received the 2018 Nobel Prize in Physics for this innovation 4 .
An attogram is 10⁻¹⁸ grams - an incredibly small unit of measurement used for weighing individual nanoparticles.
Comparing an attogram to a gram is like comparing a grain of sand to the Great Pyramid of Giza 8 .
The concept of using light to manipulate physical objects might sound like science fiction, but it's exactly what optical trapping technology accomplishes. Developed by Arthur Ashkin, who received the 2018 Nobel Prize in Physics for this innovation, optical trapping uses the subtle pressure exerted by laser beams to capture and hold microscopic particles. When light reflects or refracts, it imparts a tiny force on objects—negligible in our macroscopic world but significant for nano-sized particles with minimal mass 4 .
Think of optical trapping as a microscopic "laser tweezer" that can gently grab and hold a single nanoparticle in place. Researchers create this trap by focusing two laser beams to a precise point where their forces balance, forming what's called an "optical potential well." Any particle that enters this region finds itself pushed back toward the center regardless of which direction it moves, effectively becoming suspended in three-dimensional space 4 8 . This technology enables scientists to study individual nanoparticles without the interference of surfaces or containers that might alter their properties—a capability that has proven especially valuable for investigating atmospheric particles and their role in climate science 4 .
While optical trapping solves the problem of isolating nanoparticles, we still need a method to analyze them. This is where laser-induced breakdown spectroscopy (LIBS) comes into play. The technique works by firing a powerful, pulsed laser at the trapped nanoparticle—this laser is separate from the one used for trapping. When this analyzing laser strikes the nanoparticle, it generates an extremely hot microplasma (around 10,000°C) that vaporizes and excites the particle's atoms 3 5 .
As this miniature plasma cools, each element within the nanoparticle emits light at characteristic wavelengths—essentially creating a unique "fingerprint spectrum" for that particle. By collecting and analyzing this light with a spectrometer, researchers can identify the exact elemental composition of the nanoparticle 3 . For example, copper atoms emit different colors of light than iron or zinc atoms when excited in a plasma. What makes this approach particularly powerful is that it provides panoramic spectra from ultraviolet to infrared in a single laser shot, revealing the complete elemental makeup of the nanoparticle simultaneously 8 .
To appreciate the sensitivity of this technique, we need to understand the incredibly small masses involved. An attogram is 10⁻¹⁸ grams—an almost inconceivably tiny unit of measurement. To put this in perspective, comparing an attogram to a gram is like comparing the mass of a small grain of sand to the mass of the entire Great Pyramid of Giza! 8
When analyzing nanoparticles in the 25-100 nanometer size range, the total mass of material being examined falls precisely into this attogram realm. Traditional analytical techniques struggle at this scale because they simply don't have the sensitivity to detect such minimal amounts of material. The combination of optical trapping and LIBS has shattered these limitations, achieving detection limits as low as 57 attograms—a record-breaking sensitivity that opens up entirely new possibilities for studying matter at the ultimate limits of mass 8 .
The pioneering work developing this methodology comes from Javier Laserna and his team at the University of Málaga in Spain, who received the 2018 Lester W. Strock Award for their innovations 8 . Their approach, known as OC-OT-LIBS (Optical Catapulting-Optical Trapping-Laser Induced Breakdown Spectroscopy), combines three sophisticated techniques into one integrated platform 5 .
The process begins with optical catapulting—creating an aerosol of individual nanoparticles from a powder sample. Researchers place a nanopowder on a microscope slide and fire a laser pulse from beneath it. The shockwave generated by this pulse launches nanoparticles upward, creating a highly dispersed aerosol 8 .
Next comes the optical trapping phase. A continuous-wave laser beam propagates upward through the system, tightly focused by a microscope objective. The radiation pressure created by this laser counteracts gravity and air currents, allowing individual nanoparticles to be trapped within a specialized cuvette that protects them from laboratory air movements. The trapping process requires patience—it typically takes 3-5 minutes for a single nanoparticle to settle into the trap, depending on factors like particle size and optical properties 8 .
Once a nanoparticle is securely trapped, the analysis phase begins. A separate analyzing laser pulse (typically at 1064 nm wavelength with about 200 millijoules of energy) is fired perpendicular to the trapping beam, directly striking the suspended nanoparticle. The resulting microplasma emits characteristic light that is collected by a spectrometer and analyzed 8 .
This sophisticated approach required solving several significant technical challenges. Perhaps the most tricky aspect was discriminating the faint signal from the nanoparticle against the much stronger background signal from the air plasma. The researchers developed two clever strategies to address this:
By waiting a few microseconds after the laser pulse before collecting spectral data, the strong ionic spectral lines from nitrogen and oxygen in the air fade significantly, allowing the nanoparticle's signature to emerge 8 .
By focusing the analytical laser slightly ahead of the particle's position, the most intense air plasma lines form off-axis from where the nanoparticle signal is collected 8 .
Another challenge was ensuring that only a single nanoparticle was being analyzed at a time, as the optical trap could potentially capture clusters of particles. Researchers addressed this by carefully adjusting the laser power and manipulating the beam to cause larger particles or clusters to fall out of the trap, leaving only individual nanoparticles for analysis 8 .
| Parameter | Specification | Purpose/Function |
|---|---|---|
| Trapping Laser | Continuous-wave (CW) laser, propagating upward | Counteracts gravity and air currents to trap particles |
| Analyzing Laser | 1064 nm, ~200 mJ/pulse, pulsed | Generates plasma from single trapped nanoparticle |
| Focusing Objective | 20× magnification, 0.4 numerical aperture | Tightly focuses trapping laser to create optical potential well |
| Containment | Spectrophotometry cuvette | Protects trapped nanoparticles from air currents |
| Typical Trapping Time | 3-5 minutes | Time required for single nanoparticle to settle into trap |
The success of this methodology is vividly demonstrated in the spectral data obtained from individual nanoparticles. When the analyzing laser strikes a trapped nanoparticle, the resulting plasma emits a complex spectrum containing numerous characteristic emission lines that serve as unique fingerprints for each element present 8 .
Researchers have successfully applied this technique to both mono-elemental particles (like pure copper or nickel) and multi-elemental particles (such as ferrite particles composed of three or four different elements). In one notable experiment, the team detected unprecedented sensitivity of 57 attograms using 25-nm copper particles. Even more impressively, they achieved simultaneous multi-element detection in complex nanoparticles like CuFe₂O₄ and CuZn₁₋ₓFe₂O₄ spinel samples, where all metal components were identified from a single particle at the sub-femtogram level 5 .
| Nanoparticle Type | Size Range Tested | Elements Detected | Achieved Detection Limit |
|---|---|---|---|
| Copper (Cu) | 25-100 nm | Copper | 57 attograms |
| Nickel (Ni) | ~100 nm | Nickel | Demonstrated, specific LOD not provided |
| Silica (SiO₂) | ~100 nm | Silicon, Oxygen | Demonstrated, specific LOD not provided |
| Ferrite Particles | Sub-micron | Cu, Fe, O, (Zn) | Sub-femtogram simultaneous detection |
| Quaternary Bronze | Micro- and nanometer | Multiple metal components | Successful multi-element analysis |
Beyond simple elemental identification, this technique has revealed fascinating new insights into the fundamental physics of laser-nanoparticle interactions. By combining plasma imaging with time-resolved spectroscopy, researchers discovered that the excitation mechanisms differ significantly between bulk materials, aerosol streams, and individual trapped nanoparticles 5 .
In single-particle analysis, the interaction follows a distinct pathway: the analyzing laser first creates an air plasma, which then transfers energy to the nanoparticle, causing its dissociation and atomization. This energy transfer from the air plasma to the particle represents a dual role for the surrounding air—serving as both the initial excitation source and the medium for atomization 5 .
The research team quantified this energy transfer using Hess-like cycles to calculate the total energy absorbed by nanoparticles. Their findings revealed that the excitation efficiency in single-particle analysis depends exponentially on the energy of the excited states to which electrons are promoted. This understanding has important implications for optimizing analytical sensitivity in nanoparticle studies 5 .
The sensitivity achieved through this methodology represents a landmark in optical spectroscopy. The reported detection limit of 57 attograms for 25-nm copper nanoparticles sets a new standard for the field 8 . To appreciate this achievement, consider that conventional LIBS analysis of trace elements in solid matrices typically achieves limits in the parts-per-million (ppm) range—adequate for bulk analysis but utterly insufficient for single nanoparticles.
The extraordinary sensitivity arises from several factors unique to the single-particle approach. Since each laser pulse analyzes just one nanoparticle, there's no signal dilution from a surrounding matrix. Additionally, the plasma generated from a single nanoparticle exhibits different physical traits compared to bulk material plasmas, with more efficient excitation processes 5 .
Researchers have further discovered that the emission sensitivity in single-particle LIBS depends strongly on particle size, with smaller particles requiring more sophisticated detection strategies. This understanding has prompted investigations into hyperspectral approaches that could capture more photons from the plasma and potentially achieve even lower detection limits in the future 8 .
Conducting research at the intersection of optical trapping and laser-induced breakdown spectroscopy requires specialized equipment and materials. The table below outlines key components of the experimental setup and their functions in these sophisticated experiments.
| Equipment/Material | Function/Role in Experiment |
|---|---|
| Continuous-Wave (CW) Laser | Creates optical trap to capture and levitate individual nanoparticles 8 |
| Pulsed Laser System | Generates analyzing pulses for optical catapulting and plasma creation 8 |
| High-Numerical-Aperture Microscope Objective | Focuses trapping laser to create tight optical potential well 8 |
| Spectrophotometry Cuvette | Contains nanoparticle aerosol, protecting it from air currents 8 |
| Spectrometer with CCD Detector | Collects and resolves characteristic light emission from plasma 5 8 |
| Optical Delay Generator | Controls precise timing between laser pulse and signal acquisition 8 |
| Nanoparticle Powders | Sample materials for analysis (metals, metal oxides, functionalized particles) 8 |
| Precision XYZ Stages | Enables exact positioning of optical components and sample 8 |
The experimental setup requires precise alignment of multiple optical components:
Proper sample preparation is critical for successful experiments:
The ability to characterize individual nanoparticles at the attogram level opens exciting possibilities across numerous scientific disciplines. In environmental science, researchers can now analyze atmospheric particulate matter one particle at a time, tracking their evolution and interactions in real-time. This is particularly valuable for understanding phenomena like cloud formation and the health impacts of air pollution 4 . The technology enables scientists to capture single aerosol particles and study their reactions to changing humidity and pollutant gases while suspended in their native environment 4 .
Analysis of atmospheric particles for climate and pollution studies
Quality control for drug nanoparticles and delivery systems
Development and verification of advanced nanomaterials
In the pharmaceutical industry, where drug nanoparticles are increasingly used for improved drug delivery, this technique offers unprecedented quality control capabilities. Researchers can analyze functionalized nanoparticles synthesized in very small quantities for biological research—samples that are difficult or impossible to study with conventional technologies requiring larger amounts of material 8 . The specific mention of "drug nanoparticle delivery" in patent literature highlights the growing importance of nanomedicines that could benefit from this analytical approach 7 .
The methodology also shows great promise for materials science, where complex multi-element nanoparticles are designed for applications ranging from sensing to super-resolution imaging. The ability to verify the composition of individual specimens in unknown samples ensures that minor variations in materials don't substantially alter their efficiency in advanced applications 5 .
While the current OC-OT-LIBS platform represents a major advance, researchers are already working on next-generation improvements. The relatively low sampling efficiency—capturing approximately one particle every three minutes—currently limits throughput for analyzing large numbers of particles 8 .
To address this limitation, scientists are developing alternative approaches that would analyze nanoparticles in a flow system. This would involve creating a suspension of nanoparticles, generating an aerosol of microdroplets, and then drying them to allow analysis "on the fly" using a time-of-flight configuration with LIBS detection. Though still in early development, this approach could significantly improve sample throughput for single-nanoparticle analysis 8 .
Other innovations focus on improving photon collection efficiency, which remains challenging due to the 4π emitting geometry of plasma and significant photon losses at spectrometer entrance slits. Research into hyperspectral approaches using appropriate filter sets could potentially capture more photons from the plasma, leading to even better detection limits 8 .
As these technical improvements mature, we can anticipate broader adoption of single-nanoparticle analysis across industrial and research settings. The ongoing refinement of ultra-fast imaging techniques, including pump-probe shadow imaging and four-dimensional ultrafast electron microscopy, will further enhance our ability to observe laser-nanoparticle interactions with exceptional temporal and spatial resolution 9 .
The marriage of optical trapping with laser-induced plasma imaging has opened what was once an impenetrable black box—the world of individual nanoparticles. By enabling researchers to capture, isolate, and analyze specks of matter weighing mere attograms, this technology has pushed the boundaries of analytical science to previously unimaginable extremes.
What makes this development particularly exciting is its dual impact: it simultaneously provides practical solutions for today's scientific challenges while opening new pathways for fundamental discovery. As research teams like Laserna's continue to refine these methods, we move closer to a comprehensive understanding of the nanoscale world—one single particle at a time.
The ability to characterize attogram masses in optically trapped nanoparticles represents more than just a technical achievement; it offers a new lens through which we can examine and understand the building blocks of our material world. From developing more effective medicines to designing novel materials with tailored properties, this technology promises to illuminate previously invisible details that will shape the future of science and technology.
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