We thank the University of Hyderabad for support through the Institute of Eminence (IoE) project UOH/IOE/RC1/RC1-2016. The IoE grant obtained vide notification F11/9/2019-U3(A) from the MHRD, India. DRDO, India is acknowledged for funding support through ACRHEM [[#ERIP/ER/1501138/M/01/319/D(R&#38;D)]. We acknowledge the School of Physics, UoH, for the FESEM characterization and XRD facilities. We would like to extend our sincere gratitude to Prof SVS Nageswara Rao and his group for their valuable collaboration contributions and support. We would like to express our appreciation to past and present lab members Dr. P Gopala Krishna, Dr. Hamad Syed, Dr. Chandu Byram, Mr. S Sampath Kumar, Ms. Ch Bindu Madhuri, Ms. Reshma Beeram, Mr. A Mangababu, and Mr. K Ravi Kumar for their invaluable support and assistance during and after the laser ablation experiments in the lab. We acknowledge the successful collaboration of Dr. Prabhat Kumar Dwivedi, IIT Kanpur.

A typical protocol flowchart of the application of ultrafast ablated NPs or NSs in the trace detection of molecules via SERS is shown in Figure 1A.
1. Synthesizing metal NPs/NSs
NOTE: Depending on the requirement/application, choose the target material, the surrounding liquid, and the laser ablation parameters. 
Here: 
Target materials: Ag 
Surrounding liquid: 10 mL of DI 
Laser parameters: 35...

Synthesizing Nanoparticles and Nanostructures via Laser Ablation

The authors have nothing to disclose.

In ultrasonication cleaning, the material to be cleaned is immersed in a liquid and high-frequency sound waves are applied to the liquid using an ultrasonic cleaner. The sound waves cause the formation and implosion of tiny bubbles in the liquid, generating intense local energy and pressure that dislodge and remove dirt and other contaminants from the surface of the material. In laser ablation, a Brewster polarizer and a half-wave plate combination were used to tune the laser energy; the polarizer is typically placed bef...

Ultrafast laser ablation is a rapidly evolving field of laser-material interactions. High-intensity laser pulses with pulse durations in the femtosecond (fs) to picosecond (ps) range are used to generate precise material ablation. Compared to nanosecond (ns) laser pulses, ps laser pulses can ablate materials with higher precision and accuracy due to their shorter pulse duration. They can generate less collateral damage, debris, and contamination of the ablated material due to fewer thermal effects. However, ps lasers are typically more expensive than ns lasers and need specialized expertise for operation and maintenance. The ultrafast laser pulses enable precise control over the energy deposition, which leads to highly localized and minimized thermal damage to the surrounding material. Additionally, ultrafast laser ablation can lead to the generation of unique nanomaterials (i.e., surfactants/capping agents are not obligatory during the production of nanomaterials). Therefore, we can term this a green synthesis/fabrication method1,2,3. The mechanisms of ultrafast laser ablation are intricate. The technique involves different physical processes, such as (a) electronic excitation, (b) ionization, and (c) the generation of a dense plasma, which results in the ejection of material from the surface4. Laser ablation is a simple single-step process to produce nanoparticles (NPs) with high yield, narrow size distribution, and nanostructures (NSs). Naser et al.5 conducted a detailed review of the factors influencing the synthesis and production of NPs through the laser ablation method. The review covered various aspects, such as the parameters of a laser pulse, focusing conditions, and the ablation medium. The review also discussed their impact on producing a wide range of NPs using the laser ablation in liquid (LAL) method. The laser-ablated nanomaterials are promising materials, with applications in various fields such as catalysis, electronics, sensing, and biomedical, water splitting applications6,7,8,9,10,11,12,13,14.
Surface-enhanced Raman scattering (SERS) is a powerful analytical sensing technique that significantly enhances the Raman signal from probe/analyte molecules adsorbed onto metallic NSs/NPs. SERS is based on the excitation of surface plasmon resonances in metallic NPs/NSs, which results in a significant rise in the local electromagnetic field near the metallic nano-features. This enhanced field interacts with the molecules adsorbed on the surface, significantly enhancing the Raman signal. This technique has been used to detect various analytes, including dyes, explosives, pesticides, proteins, DNA, and drugs15,16,17. In recent years, significant progress has been made in the development of SERS substrates, including the use of differently shaped metallic NPs18,19 (nanorods, nanostars, and nanowires), hybrid NSs20,21 (a combination of the metal with other materials&#160;such as Si22,23, GaAs24, Ti25, graphene26, MOS227, Fe28, etc.), as well as flexible substrates29,30 (paper, cloth, nanofiber, etc.). Developing these new strategies in the substrates has opened up new possibilities for using SERS in various real-time applications.
This protocol discusses the fabrication of Ag NPs using a ps laser at different wavelengths and Ag-Au alloy NPs (with different ratios of Ag and Au targets) fabricated using laser ablation technique&#160;in distilled water. Additionally, silicon micro/nanostructures are created using an fs laser on silicon in the air. These NPs and NSs are characterized using ultraviolet (UV)-visible absorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), and field emission scanning electron microscopy (FESEM). Furthermore, the preparation of SERS substrates and analyte molecules are discussed, followed by the collection of Raman and SERS spectra of the analyte molecules. Data analysis is performed to determine the enhancement factor, sensitivity, and reproducibility of the laser-ablated NPs/NSs as potential sensors. Additionally, typical SERS studies are discussed, and the SERS performance of hybrid substrates is evaluated. Specifically, it has been found that the promising gold nanostars&#39; SERS sensitivity can be enhanced approximately 21 times by using laser-structured silicon instead of plain surfaces (such as Si/glass) as a base.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Alloys</td>
			<td>Local goldsmith</td>
			<td>N/A</td>
			<td>99% pure</td>
		</tr>
		<tr>
			<td>Axicon</td>
			<td>Thorlabs</td>
			<td>N/A</td>
			<td>100, IR range, AR coated, AX1210-B</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Supelco, India</td>
			<td>CAS No. 64-17-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Femtosecond laser</td>
			<td>femtosecond&#160; (fs)&#160; laser amplifier&#160; Libra HE, Coherent</td>
			<td>N/A</td>
			<td>Pulse duraction 50 fs; 
			wavelenngth 800 nm; 
			Rep rate 1 KHz; 
			Pulse Energy: 4 mJ</td>
		</tr>
		<tr>
			<td>FESEM</td>
			<td>Carl ZEISS, Ultra 55</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Gatan DM3</td>
			<td>www.gatan.com</td>
			<td>Gatan Microscopy Suite 3.x</td>
			<td></td>
		</tr>
		<tr>
			<td>Gold target&#160;</td>
			<td>Sigma-Aldrich, India</td>
			<td></td>
			<td>99% pure</td>
		</tr>
		<tr>
			<td>HAuCl4.3H2O</td>
			<td>Sigma-Aldrich, India</td>
			<td>CAS No. 16961-25-4</td>
			<td></td>
		</tr>
		<tr>
			<td>High resolution translational stages</td>
			<td>Newport SPECTRA PHYSICS GMBI</td>
			<td>N/A</td>
			<td>M-443 High-Performance Low-Profile Ball Bearing Linear Stage; 
			The stage is only 1 inch high, and has 2 inches of travel.&#160;</td>
		</tr>
		<tr>
			<td>Micro Raman</td>
			<td>Horiba LabRAM</td>
			<td>N/A</td>
			<td>Grating-1,800 and 600 grooves/mm; 
			Wavelength of excitation-785 nm,632 nm, 532 nm, 325 nm; 
			Objectives 10x, 20x, 50 x, 100x; 
			CCD detector</td>
		</tr>
		<tr>
			<td>Mirrors</td>
			<td>Edmund Optics</td>
			<td>N/A</td>
			<td>Suitable mirrors for specific wavelength of laser</td>
		</tr>
		<tr>
			<td>Motion controller</td>
			<td>NEWPORT SPECTRA PIYSICS GMBI</td>
			<td>N/A</td>
			<td>ESP300 Controller-3 axes control</td>
		</tr>
		<tr>
			<td>Origin</td>
			<td>www.originlab.com</td>
			<td>Origin 2018</td>
			<td></td>
		</tr>
		<tr>
			<td>Picosecond laser</td>
			<td>EKSPLA 2251</td>
			<td>N/A</td>
			<td>Pulse duraction 30ps; 
			wavelenngth 1064 nm, 532 nm, 355 nm; 
			Rep rate 10 Hz; 
			Pulse Energy: 1.5 to 30 mJ</td>
		</tr>
		<tr>
			<td>Planoconvex lens</td>
			<td></td>
			<td>N/A</td>
			<td>focal length 10 cm</td>
		</tr>
		<tr>
			<td>Raman portable</td>
			<td>i-Raman plus,&#160; B&#38;W Tek, USA</td>
			<td>N/A</td>
			<td>785 nm, ~ 100 &#181;m laser spot&#160; fiber optic probe excitation and collection</td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>Macwin India Ltd.</td>
			<td></td>
			<td>1&#8211;10 &#937;-cm, p (100)-type</td>
		</tr>
		<tr>
			<td>Silver salt (AgNO3)</td>
			<td>Finar, India</td>
			<td>CAS No. 7783-90-6&#160;</td>
			<td></td>
		</tr>
		<tr>
			<td>Silver target</td>
			<td>Sigma-Aldrich, India</td>
			<td>CAS NO 7440-22-4</td>
			<td>99% pure</td>
		</tr>
		<tr>
			<td>TEM</td>
			<td>Tecnai TEM</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>TEM grids</td>
			<td>Sigma-Aldrich, India</td>
			<td>TEM-CF200CU</td>
			<td>Copper Grid Carbon Coated&#160; 200 mesh</td>
		</tr>
		<tr>
			<td>Thiram</td>
			<td>Sigma-Aldrich, India</td>
			<td>CAS No. 137-26-8</td>
			<td></td>
		</tr>
		<tr>
			<td>UV</td>
			<td>Jasco V-670</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>XRD</td>
			<td>Bruker D8 advance</td>
			<td>N/A</td>
			<td></td>
		</tr>
	</tbody>
</table>

ultrafast laser-ablated nanoparticles and nanostructures for surface-enhanced raman scattering-based sensing applications

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields such as sensing, catalysis, and medicine. The exceptional feature of this technique is the formation of nanoparticles (colloids) and nanostructures (solids) in a single experiment with ultrashort laser pulses. We have been working on this technique for the past few years, investigating its potential using the surface-enhanced Raman scattering (SERS) technique in hazardous materials sensing applications. Ultrafast laser-ablated substrates (solids and colloids) could detect several analyte molecules at the trace levels/mixture form, including dyes, explosives, pesticides, and biomolecules. Here, we present some of the results achieved using the targets of Ag, Au, Ag-Au, and Si. We have optimized the nanostructures (NSs) and nanoparticles (NPs) obtained (in liquids and air) using different pulse durations, wavelengths, energies, pulse shapes, and writing geometries. Thus, various NSs and NPs were tested for their efficiency in sensing numerous analyte molecules using a simple, portable Raman spectrometer. This methodology, once optimized, paves the way for on-field sensing applications. We discuss the protocols in (a) synthesizing the NPs/NSs via laser ablation, (b) characterization of NPs/NSs, and (c) their utilization in the SERS-based sensing studies.

Author Spotlight: Advancements and Applications in Nanoparticle Synthesis Through Laser Ablation in Liquids 

Ultrafast Laser-Ablated Nanoparticles and Nanostructures for Surface-Enhanced Raman Scattering-Based Sensing Applications

Laser ablation and liquids technique focuses on synthesizing nanoparticles and nanostructures by ablating a target material in liquid or gas environment, forming ecofriendly nanoparticles without any surfactants. This research scope involves refining the laser parameters, analyzing nanoparticle properties, exploring synthesis mechanisms, and discovering applications in various fields such as catalysis, sensing, electronics, energy storage, and biomedical imaging. Recent developments in laser ablation for nanomaterial fabrication involve tailoring the nanoparticle sizes, shapes, and composition, scalable production methods, hybrid nanomaterial synthesis, and surface functionalization.These advances offer improved stability, extended shelf life, high scalability that is up to few grams per hour, and commercially viable colloidal nanoparticles and surface nanostructures production for diverse applications. In recent times, various other improvisations such as ultrafast vessel beam and vortex beam ablation processes have been implemented to generate exotic nanostructures, as well as nanoparticles, with tuneable shapes, sizes, and topographies. Technologies advancing nanoparticle synthesis via ultrafast laser ablation include molecular dynamic simulations for understanding the formation of these nanoparticles.Precise control for tailoring nanoparticle size, shape, and composition in C.2 TEM characterization and real time monitoring techniques. The experimental challenges involved are automation techniques through improved instrumentation and technology, addressing the initial establishment cost of the laser ablation technique itself, ensuring proper sample preparation, controlling the contamination, and announcing data analysis and interpretation for increased accuracy and reliability of the results. In our laboratory, we have investigated diverse materials such as gold, silver, zinc, titanium, copper, silicone, gallium arsenide, germanium, graphene oxide, Hafnium, and various biome metallic, trimetallic, and plus monoxides.Laser ablation experiments were conducted in different environments using various focusing conditions for precise material processing and nanoparticle synthesis.

Silver NPs were synthesized via ps laser ablation in liquid technique. Here, a ps laser system with a pulse duration of ~30 ps operating at a 10 Hz repetition rate and with a wavelength of one of 355, 532, or 1,064 nm was used. The input pulse energy was adjusted to 15 mJ. The laser pulses were focused using a plano-convex lens with a focal length of 10 cm. The laser focus should be exactly on the material surface during laser ablation because the laser energy is most concentrated at the focal point, where it ca...

Watch this Scientific Journal Video about Advancements and Applications in Nanoparticle Synthesis Through Laser Ablation in Liquids at JoVE.com

Ultrafast laser ablation in liquid is a precise and versatile technique for synthesizing nanomaterials (nanoparticles [NPs] and nanostructures [NSs]) in liquid/air environments. The laser-ablated nanomaterials can be functionalized with Raman-active molecules to enhance the Raman signal of analytes placed on or near the NSs/NPs.

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields ...

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Engineering

Synthesizing Metal Nanoparticles and Nanostructures Using Ultrafast Laser Ablation

To begin, choose the target material and surrounding liquid. Perform ultrasonic cleaning of the target surface using acetone for 15 minutes to remove organic materials, such as oils, greases, and waxes. Subject the surface to ultrasonic cleaning with ethanol for another 15 minutes to remove polar contaminants like salts and sugars.Then clean the surface with deionized water using ultrasonic cleaning for 15 minutes to remove any residual solvents or contaminants from the sample's surface. Measure the weight of the sample before ablation, then perform a laser ablation experiment on the sample. Measure the weight of the sample again after the ablation experiment.Estimate the amount of material removed during the experiment by comparing the weight of the sample before and after ablation. Next, adjust the input laser power to approximately 150 milliwatts for picosecond laser ablation of the silver target. Combine a polarizer and a half-wave plate to adjust the laser pulse energy.Focus the laser beam onto the sample using a focusing lens to ablate the material surface. Manually adjust the laser's focus on the sample using a translation stage in the Z direction while observing the bright plasma produced and the cracking sound emanating. Next, use focusing optics for focusing the laser beam onto the sample depending on achieving different ablation depths and improving control over nanoparticles and nano structures synthesis.Position the sample on the XY stage connected to an ESP controller ensuring it moves perpendicular to the laser propagation direction. Adjust the scan speed and laser processing area to optimize the number of laser pulses interacting with the sample. During the laser ablation process, perform laser patterning while scanning the sample to achieve the desired dimensions and prevent single point ablation.Next, for laser ablation in liquid, conduct a laser ablation experiment. Monitor laser power and other settings to maintain consistency. Continuously observe the target material during the experiment ensuring the laser beam stays focused on the desired area.The absorption spectra of silver colloidal nanoparticles revealed that surface plasmin resonance peaks were observed at 420, 394, and 403 nanometers for different laser wavelengths. The normalized absorbance spectra of silver gold alloy nanoparticles with different compositions revealed a surface plasmin resonance peak shift from 410 to 519 nanometers as the gold percentage increased from zero to 100.

1.3K Views.  University of Hyderabad.  Ultrafast laser ablation in liquid is a precise and versatile technique for synthesizing nanomaterials (nanoparticles [NPs] and nanostructures [NSs]) in liquid/air environments. The laser-ablated nanomaterials can be functionalized with Raman-active molecules to enhance the Raman signal of analytes placed on or near the NSs/NPs.

Synthesizing Nanoparticles and Nanostructures via Laser Ablation

Summary