We acknowledge financial support from Research Growth Initiative (Award No. 101X340), National Science Foundation, Major Research Instrumentation Program (Grant No. PHY-1626450), Greater Milwaukee Foundation (Shaw Award) and University of Wisconsin System (Applied Research Grant).

1. Reagents Solution Preparation
<ol>
	<li>Prepare a starting protein solution by dissolving/diluting the protein of interest to the desired concentration, using a Tris buffer [20 mM tris(hydroxymethyl)aminomethane and 150 mM NaCl, pH 7.4]. 
	NOTE: The smallest protein concentration for which cross-linking leads to hydrogels depends on the protein used and is typically &#62; 1 mM.</li>
	<li>Prepare stocks of ammonium persulfate (APS) (1 M) and tris(bipyridine)ruthenium(II) chloride ([Ru(bpy)3]2+) (6.67 mM) solutions by dissolving APS and [Ru(bpy)3]2+ powders in the Tris buffer.</li>
</ol>
2. Protein-based Hydrogel Synthesis
<ol>
	<li>Fix a 23 G needle on a 1 mL syringe with a pressed plunger.</li>
	<li>Cut a 10 cm polytetrafluoroethylene (PTFE) tube (with an inner diameter of 0.022 in and an outer diameter of 0.044 in) using a razor blade. Attach the needle and syringe to one end of the PTFE tube.</li>
	<li>Insert the second end of the tube into a silane solution and fill the tube by retracting the syringe plunger. Leave the tube for ~30 min.</li>
	<li>Remove the silane solution and dry the tube with compressed air. 
	NOTE: Make sure that all of the silane solution is dried and that no residue is left in the tube.</li>
	<li>Mix the protein solution with APS and [Ru(bpy)3]2+ in a 1.5 mL tube using a constant volume ratio [e.g., 15:1:1 or 15:0.5:0.5 (v:v:v)].</li>
	<li>Vortex the photoactive solution until it is mixed completely.</li>
	<li>Centrifuge the mix at maximum speed (e.g., 14,000 x g) to remove any bubbles from the solution.</li>
	<li>Insert the open end of the treated PTFE tube into the photoactive mixture and draw the solution into the tube by retracting the syringe plunger.</li>
	<li>Place the loaded tube ~10 cm away from a 100 W mercury lamp to prevent heating it and keep it there for up to 30 min at room temperature (Figure 1B). 
	NOTE: In some cases, the exposure time to light can be as low as 30 s. Shorter gelation times are used here for fluorescent gels, to limit photobleaching.</li>
	<li>Remove the tube from the needle and cut the edges of the tube near the hydrogel ends with a razor blade.</li>
	<li>Use a blunted 24 G needle to extrude the hydrogel into the Tris solution (Figure 1C). 
	NOTE: Blunted needles are used to avoid any notches or damage to the hydrogel sample.</li>
	<li>Visually inspect the gels for any defects that might form during the extrusion or due to bubbles and discard the gels with defects.</li>
</ol>
3. Protein-based Hydrogel Attachment and Force-Clamp Rheometer Set-up
<ol>
	<li>Start the instrument control program. Turn on the voice-coil motor. Set the coil position to a value toward the end of the range (e.g., 7.5 mm). 
	NOTE: The voice-coil position is recommended to be toward the end of the maximum movement range, to maximize the possible extension of the hydrogel.</li>
	<li>Displace the hooks in the z-direction and align them at the bend in the x-direction (which is the pulling coordinate; see Figure 2B). Record the values of the micrometer screws for the x-direction.</li>
	<li>Cut 2 sterile sutures into strands of equal length (2 - 3 cm; see Figure 3A and B).</li>
	<li>Tie a loose double overhand knot into each of the strands and place the 2 loops on the hook connected to the force sensor (Figure 3C and 3D).</li>
	<li>Fill the experimental chamber with Tris buffer and transfer the hydrogel sample into the filled chamber using medical tweezers.</li>
	<li>Place the voice coil and force sensor hooks close to the solution surface and align the hooks in all directions using the x/y/z-positioning manipulators.</li>
	<li>Using medical tweezers, hang both sides of the protein hydrogel sample on the hooks connected to the voice coil and force sensor (Figure 3C).</li>
	<li>Tighten 1 suture loop around the hydrogel sample on the voice coil hook by holding both ends of the suture loop with medical tweezers and pulling them simultaneously (Figure 3D).</li>
	<li>Repeat step 3.8 for the loop connected to the force sensor (Figure 3D). 
	NOTE: Avoid an extreme tightening of the sutures to prevent any structural damage and transversal cutting of the hydrogel sample.</li>
	<li>Tighten the suture loops on the bends of each hook to prevent any slippage; use these bends as reference points to find the zero separation between the hooks in step 3.2. Cut the excess lengths of the sutures using medical scissors (Figure 3D).</li>
	<li>Move the attached hydrogel using z-manipulators along the z-axis toward the experimental chamber to immerse the hydrogel in the experimental solution.</li>
	<li>Align the hydrogel sample in y-z using the manipulators such that the gel is not under any stress.</li>
	<li>Zero the force sensor and separate the two hooks using the x-micrometer stages until the gel starts to experience force. Once this happens, slightly turn back the micrometer screw in the x-direction.</li>
	<li>Record the position of both manipulators for the voice coil motor and the sensor and use the difference between these values and the ones measured in step 3.2 to calculate the exact separation between the tethering hooks at the start of the experiment.</li>
	<li>Set the range for the slack curve to ~1.5 - 2 mm and measure the gel slack (Figure 4A). 
	NOTE: For each slack measurement, try to keep the start of the slack regime near the initial voice coil position, allowing for an optimal number of data points to fit the 2 regimes (Figure 4A). The gel length can be determined with a micron resolution using the separation between the hooks and the intersection between the 2 regimes in the slack curve (see also step 5.1). As the force sensor might drift with time due to variations in the experimental conditions, the part of the slack curve where the gel is not under force reports on this possible drift. The program controlling the instrument compensates automatically for this difference when sending the set-point command to the PID loop (Figure 4A inset).</li>
</ol>
4. Protein-based Hydrogel Characterization using Controlled Force-Ramp and Constant-Force Measurements
<ol>
	<li>Force-ramp experiments
	<ol>
		<li>To perform a force-ramp cycle by increasing the force at the desired loading rate (e.g., 0.01 mN/s), input the starting and final forces and the duration of the protocol as a flipped &#34;V&#34;. Then, hold the gel at 0 mN (or low force) for &#62; 200 s to allow the protein domains to refold and the gel elasticity to recover.</li>
		<li>Save the trace.</li>
	</ol>
	</li>
	<li>Constant-force experiments
	<ol>
		<li>Perform a constant-force protocol by applying a low force (e.g., 0.1 mN) for 30 s and then increase the force to a constant force (e.g., 1 mN) for a defined amount of time (e.g., 120 s), followed by quenching the force back to the same low value (e.g., 0.1 mN) for &#62; 300 s to allow the protein domains to refold and the gel elasticity to recover.</li>
		<li>Following the first pulse, adjust the PID settings to maximize the response time of the feedback loop (see Figure 2D). 
		NOTE: For stiff gels and for small changes in force, the response time of the loop is limited by the electronics of the force sensor and the response time of the coil, and can be as low as 5 ms7. For softer gels and large changes in force, the response time is dictated by the elasticity of the hydrogels (Figure 2D).</li>
		<li>Save the trace.</li>
	</ol>
	</li>
</ol>
5. Data Analysis
<ol>
	<li>Utilizing the measured separation between the hooks and the calculated coil position, when the gel starts to experience force (&#916;x in the Figure 4A insert), compute the gel length L using the equation: 
	L = L0 + &#8710;x 
	Here, L0 is the separation between the hooks, measured from the position of the micrometer screws before the experiment (step 3.14). 
	NOTE: For gels with low protein concentrations that do not result in a complete cross-linking, the measured length will change from trace-to-trace. Also, over long periods of time, proteins inside hydrogels might experience aging effects25, which result in an overall lengthening of the gel.</li>
	<li>Normalize the measured extension to the gel length to obtain the strain.</li>
	<li>Normalize the measured force to the transversal surface area by using the inner diameter of the tube used for the polymerization.</li>
</ol>

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The authors have nothing to disclose.

Herein, we describe a force-clamp rheometry technique to investigate the biomechanical response of low-volume protein-based hydrogels. Additionally, a protocol is provided to synthesize a uniform cylindrical low-volume protein hydrogel sample. A protocol is also presented which describes how to tie different types of protein-based hydrogels with various elasticities without causing any mechanical deformation or damage to the protein-based hydrogel samples or slippage of the gel on the hooks. The analog PID system, togeth...

Apart from having unique physical properties, protein-based hydrogels hold the promise of revolutionizing force spectroscopy by enabling the measurement of several billion molecules in one &#39;pull&#39;, thus enabling the study of proteins in crowded environments, similar to those encountered in skin and other tissues. Protein domains remain folded inside hydrogels, allowing the study of their biomechanical response to force, binding partners, and chemical conditions. Additionally, the biomechanical response of protein domains inside hydrogels resembles the response seen with single-molecule force spectroscopy techniques. For example, chemical denaturants and oxidizing agents decrease the stability of the folded state, both at the single protein domain level1,2,3 and at the macroscopic level4,5,6,7. Similarly, osmolytes increase the stability of single proteins8,9, leading to a decrease in the viscoelastic response&#160;of hydrogels, for the same force conditions7,10.
Several approaches have been implemented to synthesize protein-based hydrogels, by either using physical interactions11,12 or covalent cross-linking4,13. Covalent reactions allow for fixed cross-linking locations and these hydrogels&#160;can recover the initial state upon a removal of the mechanical or chemical perturbations. A successful approach for covalent cross-linking relies on forming covalent carbon-carbon bonds between exposed tyrosine amino acids using ammonium persulfate (APS) as an oxidant and a ruthenium (II) salt as an initiator (Figure 1)14. Upon exposure to white light, a solution of concentrated proteins can be turned into a hydrogel. By controlling when the reaction starts, the protein-APS mix can be injected into any casting form, such as polytetrafluoroethylene (PFTE) tubes (Figure 1B and 1C), allowing the use of an extremely small solution volume15. Furthermore, the use of white light to trigger the cross-linking reaction results in a limited bleaching of fluorescent proteins and allows the formulation of composite hydrogels with fluorescent markers (Figure 1). Other protein-based hydrogel formation methods use cross-linking based on the SpyTag-SpyCatcher covalent interaction16, amine cross-linking via glutaraldehyde13, or biotin-streptavidin interactions17.
Dynamic mechanical analysis (DMA) is currently a technique extensively used to study polymer-based hydrogels13,18. While DMA can apply constant force protocols to biomaterials, it requires Young&#39;s moduli over 10 kPa, and large sample volumes of more than 200 &#181;L19. Due to these limitations, protein hydrogels are generally too soft to be investigated by this technique. As engineered polyproteins are harder to synthesize than polymers, since they require a living system to produce, such high volumes are inefficient, at best4,15. Furthermore, most biological tissues are softer than 10 kPa. Several approaches were developed for biological samples, especially in the study of muscle elasticity20,21. These techniques can also operate under feedback to apply constant force but are optimized for samples with small diameters (in the micron range) exposed to force for very short times (typically less than 1 s).
Protein-based hydrogels were successfully studied with modified rheometry techniques. For example, casting the hydrogel in a ring shape allows the use of extensional rheometry to measure the change in the experienced force as a function of extension4,22. Other approaches for studying the rheological properties of protein-based hydrogels use controlled shear-stress rheometry. These techniques can also achieve low sample volume and tolerate soft materials. However, these methods lack the ability to mimic the pulling forces that cause protein unfolding in vivo, and Young&#39;s modulus is calculated based on complex theories that require various assumptions and corrections23.
We have recently reported a new approach that utilizes a small volume of proteins, polymerized inside tubes with&#160;diameters &#60; 1 mm. Our first implementation of this technique was operating in length-clamp mode, where the gel was extended following the desired protocol15. In this method, the proteins experience a continuous change in both extension and force while the domains unfold, making the data interpretation cumbersome. Recently, we have reported a new force-clamp rheometry technique, where a feedback loop can expose low-volume protein hydrogels to a predefined force protocol7 (Figure 2). An analog PID system compares the force measured by the force sensor with the set point sent from the computer and adjusts the gel extension by moving the voice coil to minimize the difference between the two inputs. This &#39;clamping&#39; of the force now allows for new types of experiments to measure the biomechanics of protein hydrogels.
In the force-ramp mode, a tethered protein hydrogel experiences a constant increase and decrease of force with time. The PID compensates for any viscoelastic deformation by changing the extension in a non-linear way, depending on the type of protein and hydrogel formulation. The main advantage of force ramp is that it allows the quantification of standard parameters, such as Young&#39;s modulus and energy dissipation, due to an unfolding and refolding of protein domains.
In constant-force mode, the applied force changes in a step-like fashion. In this mode, the gel extends and contracts elastically when the force is increased or decreased, respectively, followed by a time-dependent deformation. This viscoelastic deformation, taking place while the gel experiences a constant force, is directly related to domain unfolding/refolding. In a simplified way, this extension can be seen as the equivalent of several billion single molecule traces averaged together and measured all at once. Constant-force protocols can be used to study the creep and relaxation of protein hydrogels as a function of force and time. As a function of force, for BSA-based protein hydrogels, we have recently shown that there is a linear dependency between the elastic and viscoelastic extension and recoil with the applied strain7.
Here we detail the operation of a force-clamp rheometer using composite gels made from a mixture of protein L (8 domains24, depicted as L8) and a protein L-eGFP construct (L-eGFP), which makes the overall hydrogel fluorescent and easy to demonstrate.

<table>
	<tbody>
		<tr>
			<td>SI-KG4A force transducer</td>
			<td>World Precision Instruments (WPI)</td>
			<td>SI-KG4A</td>
			<td></td>
		</tr>
		<tr>
			<td>Linear Voice Coil Motor</td>
			<td>Equipement Solutions</td>
			<td>LFA2010</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine serum albumin</td>
			<td>Rocky Mountain Biologicals (RMBIO)</td>
			<td>BSA-AAF-1XG / 100 G</td>
			<td></td>
		</tr>
		<tr>
			<td>Trizma</td>
			<td>Sigma-Aldrich</td>
			<td>T1503-1KG</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Sigma-Aldrich</td>
			<td>S7653-1KG</td>
			<td></td>
		</tr>
		<tr>
			<td>Ammonium persulfate</td>
			<td>Sigma-Aldrich</td>
			<td>248614-100G</td>
			<td></td>
		</tr>
		<tr>
			<td>Tris(bipyridine)ruthenium(II) chloride</td>
			<td>Sigma-Aldrich</td>
			<td>544981-1G</td>
			<td></td>
		</tr>
		<tr>
			<td>EXPRESS MEDICAL SUPPLIES&#160;6-0 NYLON SUTURE 12/PK</td>
			<td>Fisher Scientific</td>
			<td>NC0395626</td>
			<td></td>
		</tr>
		<tr>
			<td>1mL Syringe Only, Luer-Lok Tip</td>
			<td>BD</td>
			<td>309628</td>
			<td></td>
		</tr>
		<tr>
			<td>Silane, Sigmacote</td>
			<td>Sigma-Aldrich</td>
			<td>SL2-25ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Microbore PTFE Tubing, 0.022&#34;ID x 0.042&#34;OD, 100 ft/roll</td>
			<td>Cole-Parmer</td>
			<td>EW-06417-21</td>
			<td></td>
		</tr>
		<tr>
			<td>Hypodermic Needle, 23 Gauge</td>
			<td>Healthcare Supply Pros</td>
			<td>305194</td>
			<td></td>
		</tr>
		<tr>
			<td>Jensen Global JG24-1.5X Red IT Dispensing Tips - 24 gauge</td>
			<td>KIMCO</td>
			<td>JG24-1.5X</td>
			<td></td>
		</tr>
		<tr>
			<td>USH-103D USHIO 100W Short Arc Mercury Lamp</td>
			<td>ALB</td>
			<td>USH-103D USHIO</td>
			<td></td>
		</tr>
		<tr>
			<td>Medical Tweezers</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Medical scissors</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Olympus</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>The computer code and CAD design of the custom parts can be made available on request to the corresponding author.</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

force-clamp rheometry for characterizing protein-based hydrogels

Here, we describe a force-clamp rheometry method to characterize the biomechanical properties of protein-based hydrogels. This method uses an analog proportional-integral-derivative (PID) system to apply controlled-force protocols on cylindrical protein-based hydrogel samples, which are tethered between a linear voice-coil motor and a force transducer. During operation, the PID system adjusts the extension of the hydrogel sample to follow a predefined force protocol by minimizing the difference between the measured and set-point forces. This unique approach to protein-based hydrogels&#160;enables the tethering of extremely low-volume hydrogel samples (&#60; 5 &#181;L) with different protein concentrations. Under force-ramp protocols, where the applied stress increases and decreases linearly with time, the system enables the study of the elasticity and hysteresis behaviors associated with the (un)folding of proteins and the measurement of standard elastic and viscoelastic parameters. Under constant-force, where the force pulse has a step-like shape, the elastic response, due to the change in force, is decoupled from the viscoelastic response, which comes from protein domain unfolding and refolding. Due to its low-volume sample and versatility in applying various mechanical perturbations, force-clamp rheometry is optimized to investigate the mechanical response of proteins under force using a bulk approach.

Figure 1A shows the scheme of the photoactive reaction used to synthesize the L-EGP/L8 hydrogel. Figure 1B shows the hydrogel mixture in the PTFE tube before and after the photoactivation. Figure 1C presents the extruded L-eGFP-L8 hydrogel inside a Tris solution. The hydrogel sample has no structural defects such as notches. Hydrogels with clearly visible damage should be discar...

Watch this Scientific Journal Video about Force-Clamp Rheometry for Characterizing Protein-based Hydrogels at JoVE.com

Force-Clamp Rheometry for Characterizing Protein-based Hydrogels

A new force-clamp rheometry technique is used to investigate the mechanical properties of low-volume protein-based hydrogel samples tethered between a voice-coil motor and a force sensor. An analog proportional-integral-derivative (PID) system allows for the &#39;clamping&#39; of the force experienced to the desired protocol.

Here, we describe a force-clamp rheometry method to characterize the biomechanical properties of protein-based hydrogels. This method uses an analog ...

force-clamp-rheometry-for-characterizing-protein-based-hydrogels

Research

JoVE Journal

Engineering

6.8K Views.  University of Wisconsin-Milwaukee. A new force-clamp rheometry technique is used to investigate the mechanical properties of low-volume protein-based hydrogel samples tethered between a voice-coil motor and a force sensor. An analog proportional-integral-derivative (PID) system allows for the 'clamping' of the force experienced to the desired protocol.

Video: Force-Clamp Rheometry for Characterizing Protein-based Hydrogels

Compact Quantum Dots for Single-molecule Imaging

Single-molecule imaging is an important tool for understanding the mechanisms of biomolecular function and for visualizing the spatial and temporal heterogeneity of molecular behaviors that underlie cellular biology 1-4. To image an individual molecule of interest, it is typically conjugated to a fluorescent tag (dye, protein, bead, or quantum dot) and observed with epifluorescence or total internal reflection fluorescence (TIRF) microscopy. While dyes and fluorescent proteins have been the mainstay of fluorescence imaging for decades, their fluorescence is unstable under high photon fluxes necessary to observe individual molecules, yielding only a few seconds of observation before complete loss of signal. Latex beads and dye-labeled beads provide improved signal stability but at the expense of drastically larger hydrodynamic size, which can deleteriously alter the diffusion and behavior of the molecule under study.
Quantum dots (QDs) offer a balance between these two problematic regimes. These nanoparticles are composed of semiconductor materials and can be engineered with a hydrodynamically compact size with exceptional resistance to photodegradation 5. Thus in recent years QDs have been instrumental in enabling long-term observation of complex macromolecular behavior on the single molecule level. However these particles have still been found to exhibit impaired diffusion in crowded molecular environments such as the cellular cytoplasm and the neuronal synaptic cleft, where their sizes are still too large 4,6,7.
Recently we have engineered the cores and surface coatings of QDs for minimized hydrodynamic size, while balancing offsets to colloidal stability, photostability, brightness, and nonspecific binding that have hindered the utility of compact QDs in the past 8,9. The goal of this article is to demonstrate the synthesis, modification, and characterization of these optimized nanocrystals, composed of an alloyed HgxCd1-xSe core coated with an insulating CdyZn1-yS shell, further coated with a multidentate polymer ligand modified with short polyethylene glycol (PEG) chains (Figure 1). Compared with conventional CdSe nanocrystals, HgxCd1-xSe alloys offer greater quantum yields of fluorescence, fluorescence at red and near-infrared wavelengths for enhanced signal-to-noise in cells, and excitation at non-cytotoxic visible wavelengths. Multidentate polymer coatings bind to the nanocrystal surface in a closed and flat conformation to minimize hydrodynamic size, and PEG neutralizes the surface charge to minimize nonspecific binding to cells and biomolecules. The end result is a brightly fluorescent nanocrystal with emission between 550-800 nm and a total hydrodynamic size near 12 nm. This is in the same size range as many soluble globular proteins in cells, and substantially smaller than conventional PEGylated QDs (25-35 nm).

We describe the preparation of colloidal quantum dots with minimized hydrodynamic size for single-molecule fluorescence imaging. Compared to conventional quantum dots, these nanoparticles are similar in size to globular proteins and are optimized for single-molecule brightness, stability against photodegradation, and resistance to nonspecific binding to proteins and cells.

Single-molecule imaging is an important tool for  understanding the mechanisms of biomolecular function and for visualizing the  spatial and temporal ...

Hyperpolarized Xenon for NMR and MRI Applications

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 T generate only a small detectable net-magnetization of the sample at room temperature 1. Hence, most NMR and MRI applications rely on the detection of molecules at relative high concentration (e.g., water for imaging of biological tissue) or require excessive acquisition times. This limits our ability to exploit the very useful molecular specificity of NMR signals for many biochemical and medical applications. However, novel approaches have emerged in the past few years: Manipulation of the detected spin species prior to detection inside the NMR/MRI magnet can dramatically increase the magnetization and therefore allows detection of molecules at much lower concentration 2.
Here, we present a method for polarization of a xenon gas mixture (2-5% Xe, 10% N2, He balance) in a compact setup with a ca. 16000-fold signal enhancement. Modern line-narrowed diode lasers allow efficient polarization 7 and immediate use of gas mixture even if the noble gas is not separated from the other components. The SEOP apparatus is explained and determination of the achieved spin polarization is demonstrated for performance control of the method.
The hyperpolarized gas can be used for void space imaging, including gas flow imaging or diffusion studies at the interfaces with other materials 8,9. Moreover, the Xe NMR signal is extremely sensitive to its molecular environment 6. This enables the option to use it as an NMR/MRI contrast agent when dissolved in aqueous solution with functionalized molecular hosts that temporarily trap the gas 10,11. Direct detection and high-sensitivity indirect detection of such constructs is demonstrated in both spectroscopic and imaging mode.

The production of hyperpolarized xenon by means of spin exchange optical pumping (SEOP) is described. This method yields a ~10000-fold enhancement of the nuclear spin polarization of Xe-129 and has applications in nuclear magnetic resonance spectroscopy and imaging. Examples of gas phase and solution state experiments are given.

Nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI) suffer from intrinsic low sensitivity because even strong external magnetic fields of ~10 ...

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Micro-masonry for 3D Additive Micromanufacturing

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.

This paper introduces a 3D additive micromanufacturing strategy (termed &#8216;micro-masonry&#8217;) for the flexible fabrication of microelectromechanical system (MEMS) structures and devices. This approach involves transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and Terahertz imaging. For about 45 years, the generation of coherent, far-infrared radiation has been accomplished using the optically pumped molecular laser. Once far-infrared laser radiation is detected, the frequencies of these laser emissions are measured using a three-laser heterodyne technique. With this technique, the unknown frequency from the optically pumped molecular laser is mixed with the difference frequency between two stabilized, infrared reference frequencies. These reference frequencies are generated by independent carbon dioxide lasers, each stabilized using the fluorescence signal from an external, low pressure reference cell. The resulting beat between the known and unknown laser frequencies is monitored by a metal-insulator-metal point contact diode detector whose output is observed on a spectrum analyzer. The beat frequency between these laser emissions is subsequently measured and combined with the known reference frequencies to extrapolate the unknown far-infrared laser frequency. The resulting one-sigma fractional uncertainty for laser frequencies measured with this technique is &#177; 5 parts in 107. Accurately determining the frequency of far-infrared laser emissions is critical as they are often used as a reference for other measurements, as in the high-resolution spectroscopic investigations of free radicals using laser magnetic resonance. As part of this investigation, difluoromethane, CH2F2, was used as the far-infrared laser medium. In all, eight far-infrared laser frequencies were measured for the first time with frequencies ranging from 0.359 to 1.273 THz. Three of these laser emissions were discovered during this investigation and are reported with their optimal operating pressure, polarization with respect to the CO2 pump laser, and strength.

We describe the generation of far-infrared radiation using an optically pumped molecular laser along with the measurement of their frequencies with heterodyne techniques. The experimental system and techniques are demonstrated using difluoromethane (CH2F2) as the laser medium whose results include three new laser emissions and eight measured laser frequencies.

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and ...

A Simple Dewar/Cryostat for Thermally Equilibrating Samples at Known Temperatures for Accurate Cryogenic Luminescence Measurements

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and a CCD spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, fast sample freezing, fast alignment of the sample, accurate and stable sample temperatures, and small size and portability of the Dewar/cryostat cryogenic unit. When coupled with the fast data acquisition rates of the CCD spectrograph, this Dewar/cryostat is capable of supporting cryogenic luminescence spectroscopic measurements on luminescent samples at a series of known, stable temperatures in the 77-300 K range. A temperature-dependent study of the oxygen quenching of luminescence in a rhodium(III) transition metal complex is presented as an example of the type of investigation possible with this Dewar/cryostat. In the context of this apparatus, a stable temperature for cryogenic spectroscopy means a luminescent sample that is thermally equilibrated with either liquid nitrogen or gaseous nitrogen at a known measureable temperature that does not vary (&#916;T &#60; 0.1 K) during the short time scale (~1-10 sec) of the spectroscopic measurement by the CCD. The Dewar/cryostat works by taking advantage of the positive thermal gradient dT/dh that develops above liquid nitrogen level in the Dewar where h is the height of the sample above the liquid nitrogen level. The slow evaporation of the liquid nitrogen results in a slow increase in h over several hours and a consequent slow increase in the sample temperature T over this time period. A quickly acquired luminescence spectrum effectively catches the sample at a constant, thermally equilibrated temperature.

A simple liquid nitrogen Dewar/cryostat apparatus comprised of a small fused silica optical Dewar, a thermocouple, and a charge-coupled device (CCD) spectrograph are described. The experiments for which this Dewar/cryostat is designed require fast sample loading, freezing, and alignment, accurate and stable sample temperatures, and small size/portability.

The design and operation of a simple liquid nitrogen Dewar/cryostat apparatus based upon a small fused silica optical Dewar, a thermocouple assembly, and ...

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...

Hybrid Printing for the Fabrication of Smart Sensors

A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, three substrates (acrylate, ceramics, and copper) are prepared. To determine the resulting material properties of these substrates, profilometer, contact angle, scanning electron microscope (SEM), and focused ion beam (FIB) measurements are done. The achievable printing resolution and suitable drop volume for each substrate are, then, found through the drop size tests. Then, layers of insulating and conductive ink are inkjet printed alternately to fabricate the target sensor structures. After each printing step, the respective layers are individually treated by photonic curing. The parameters used for the curing of each layer are adapted depending on the printed ink, as well as on the surface properties of the respective substrate. To confirm the resulting conductivity and to determine the quality of the printed surface, four-point probe and profilometer measurements are done. Finally, a measurement set-up and results achieved by such an all-printed sensor system are shown to demonstrate the achievable quality.

Here we present a protocol for the fabrication of inkjet-printed multilayer sensor structures on additively manufactured substrates and foil.

A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, ...