Uranium (VI) Ion Detection Using a Hexalysine Probe

Joe OConnell, Cody Schultz, Rebecca Lai Author

07/28/2021 Added

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Joe OConnell Chemistry REU presentation summer 2021

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[00:00:00.380]Hello, I'm Joe O'Connell.
[00:00:02.100]I worked with graduate student Cody Schultz
[00:00:04.070]and professor Rebecca Lai on this product,
[00:00:06.310]entitled Uranium VI Ion Detection
[00:00:08.750]Using a Hexalysine Probe.
[00:00:12.060]Uranium contamination is a significant issue
[00:00:14.490]in parts of the United States,
[00:00:16.150]especially across the Great Plains and parts of California.
[00:00:19.340]Uranium is toxic and radioactive.
[00:00:21.210]It can be absorbed into bones and cause radiation damage,
[00:00:23.630]as well as being difficult for the kidneys to filter out,
[00:00:25.970]leading to kidney issues.
[00:00:27.640]The EPA has set the maximum contamination level
[00:00:30.050]at 30 micrograms per liter, or 126 nanomolar.
[00:00:33.704]These maps show in dark red where the uranium level is above
[00:00:35.982]the maximum contamination level as set by the EPA.
[00:00:39.740]In this project, Lai Lab is attempting to design an easy,
[00:00:42.940]more affordable uranium concentration sensor.
[00:00:45.730]The current methods are ICP mass spectrometry
[00:00:47.930]or alpha decay sensors.
[00:00:49.550]ICP-MS is expensive, must be done in a lab, and uses argon.
[00:00:54.210]Alpha decay lacks specificity
[00:00:55.610]for the specific radioactive species
[00:00:57.400]that is emitting this alpha radiation.
[00:00:59.800]This summer, my project was testing
[00:01:01.380]if a chain of six lysine residues
[00:01:03.400]could bind uranium VI ions in water samples
[00:01:05.840]in a way that changes probe flexibility
[00:01:07.690]to allow for the detection of uranium.
[00:01:10.060]This was done by taking a probe with six lysine molecules
[00:01:12.990]and a conductive methylene blue label at the end,
[00:01:15.030]and attaching it to a gold electrode.
[00:01:17.290]Uranium should bind to the lysine molecules
[00:01:19.260]and decrease probe flexibility
[00:01:20.900]and decrease the current going through the probe.
[00:01:23.210]We used alternating current voltammetry
[00:01:25.020]to measure these currents.
[00:01:26.650]This method applies a linearly increasing voltage
[00:01:29.100]with an underlying alternating voltage,
[00:01:30.920]and measures the current through each probe.
[00:01:33.210]The frequency of the underlying alternating voltage
[00:01:35.500]is what I am referring to when I mention frequency.
[00:01:37.950]In water, the uranium VI ion binds with two oxygen molecules
[00:01:42.420]to create the UO two, two plus ion, or uranyl.
[00:01:46.092]The uranyl ion has a unique electron field
[00:01:48.560]that allows for possible binding to lysine.
[00:01:51.290]Lysine is positively charged as shown on the right,
[00:01:53.700]with the long carbon side chain
[00:01:55.170]ending in a positively charged amine group.
[00:01:57.320]The abbreviation for lysine is K.
[00:01:59.610]Uranium VI has a plus six charge, and in water,
[00:02:02.460]those two oxygen atoms that bind to it
[00:02:04.300]each have a negative two charge,
[00:02:05.600]creating the UO two two plus ion.
[00:02:08.060]This ion is still positively charged however,
[00:02:10.060]and a logical question would be,
[00:02:11.800]how do you expect a positive charge
[00:02:13.240]to bind to a positive charge?
[00:02:14.690]I thought like charges repel.
[00:02:17.120]Like charges do repel,
[00:02:18.260]but these oxygen atoms have a lot of electrons around them
[00:02:20.560]that are negatively charged,
[00:02:21.700]which allows them to bind to the positively charged lysines.
[00:02:25.050]This should work in favor if this probe is a good sensor,
[00:02:28.100]as it should not only be able to respond to uranium,
[00:02:30.270]but also ignore other metal ions
[00:02:32.150]that do not have this unique property.
[00:02:34.490]This figure shows the experimental setup.
[00:02:36.500]In the middle is a glass silver, silver chloride
[00:02:38.560]reference electrode,
[00:02:39.600]on the left is a two millimeter diameter
[00:02:41.720]gold disc working electrode,
[00:02:43.060]and on the right is a platinum counter electrode.
[00:02:45.550]Prior to any electrochemical analysis,
[00:02:48.020]the beaker is partially filled with a buffer
[00:02:49.880]or electrolyte solution,
[00:02:51.060]so that the ends of all three electrodes are in solution.
[00:02:53.700]Also, there are six working gold electrodes
[00:02:55.610]when running experiments.
[00:02:57.230]Before and after each experiment,
[00:02:58.700]the gold working electrodes were electrochemically cleaned
[00:03:01.840]in .5 molar sodium hydroxide,
[00:03:03.810]polished using diamond suspensions of different sizes,
[00:03:06.780]sonicated in water, and electrochemically cleaned
[00:03:09.200]in .5 molar sulfuric acid.
[00:03:11.660]After the cleaning process,
[00:03:12.810]the real area was electrochemically determined
[00:03:16.050]using cyclic voltammetry in .05 molar sulfuric acid.
[00:03:20.620]This allows for the detection of other things
[00:03:22.530]still on the surface,
[00:03:23.550]which would result in low areas,
[00:03:25.510]or scratches and indentations in the surface,
[00:03:27.640]which would result in high areas.
[00:03:29.630]We found that this system preferred the rougher surfaces.
[00:03:32.730]The peptide probes were mobilized onto the electrode
[00:03:35.490]by posting a 10 microliter droplet
[00:03:37.270]containing 20 micromolar K6
[00:03:39.370]immobilized onto the electrode for an hour.
[00:03:42.430]The electrode was then placed in a solution
[00:03:44.290]with two micromolar mercaptohexanol
[00:03:46.290]for 16 to 18 hours overnight
[00:03:48.970]prior to running the experiment.
[00:03:50.720]The probe coverages as determined by ACV
[00:03:53.110]ranged from four to seven times 10 to the 11th
[00:03:56.080]molecules per square centimeter.
[00:03:58.950]This figure shows the alternating current voltammetry scans
[00:04:02.540]at 100 Hertz in buffer before the addition of uranium,
[00:04:06.620]with the uranium, and in buffer
[00:04:08.530]after the regeneration in 1% SDS.
[00:04:12.090]The way that this graph is analyzed
[00:04:13.670]is by taking the percent difference of the peak currents
[00:04:16.080]before in Phys2 minus and with uranium.
[00:04:18.630]This is referred to as signal suppression.
[00:04:21.110]This figure clearly shows that the signal was suppressed
[00:04:24.000]by the uranium ion, as the current never went as high,
[00:04:26.800]and that the probe regenerated to near the old current
[00:04:29.460]after treatment for four minutes in 1% SDS.
[00:04:33.760]This plot shows how the signal suppression changes
[00:04:36.160]as the frequency changes.
[00:04:37.920]The signal suppression goes up at higher frequencies.
[00:04:40.610]However, an issue arises at these higher frequencies.
[00:04:43.600]On the right is an ACB scan in buffer.
[00:04:46.040]This is hard to analyze,
[00:04:47.050]as both sides do not reach zero current,
[00:04:48.950]and there's not as clean of a peak.
[00:04:50.680]This is because this is at 300 Hertz.
[00:04:53.520]For this reason, we go with 100 Hertz
[00:04:55.630]as the optimal frequency.
[00:04:57.190]This noise is caused by the way
[00:04:58.530]that methylene blue conducts electricity.
[00:05:00.920]It exchanges two electrons when it conducts,
[00:05:02.960]and this process takes some time.
[00:05:04.810]At frequencies above 100 Hertz,
[00:05:06.400]some of the methylene blue is not able to conduct
[00:05:09.560]and then reattract the two electrons,
[00:05:11.420]resulting in less clean of peaks.
[00:05:13.580]This is a calibration curve I've taken
[00:05:15.350]by titrating in uranium overtime.
[00:05:17.440]The signal suppression increased
[00:05:18.780]with the concentration of uranium added.
[00:05:20.960]This system is capable of detecting the presence of uranium
[00:05:23.550]at concentrations as low as 25 nanomolar,
[00:05:26.390]far below the EPA maximum contamination level
[00:05:28.930]of 126 nanomolar.
[00:05:31.740]The probe showed good metal ion specificity.
[00:05:34.430]This experiment involved adding an ion cocktail,
[00:05:36.910]then adding uranium in afterwards.
[00:05:39.320]The ions in the cocktail were one micromolar calcium,
[00:05:42.380]magnesium, potassium, zinc, copper, manganese, iron,
[00:05:46.380]cadmium, chromium, nickel, and tin.
[00:05:49.490]The signal suppression from the ion cocktail
[00:05:51.240]was much lower than the uranium.
[00:05:53.580]This indicates good metal ion specificity.
[00:05:56.790]There was an interfering species in tap water.
[00:05:59.240]A similar test in our lab made tap water,
[00:06:01.330]as well as Lincoln tap water from the faucet
[00:06:03.890]did not show as promising of results.
[00:06:06.280]These were run in 50% of the test sample,
[00:06:08.500]and 50% phase four minus,
[00:06:10.300]which is a two time strength version of our phase two minus.
[00:06:13.350]This synthetic tap consisted of DI water,
[00:06:16.150]with iron III, magnesium, phosphorus,
[00:06:18.170]potassium, sodium, zinc, copper, and manganese.
[00:06:21.350]They have significant signal suppression
[00:06:22.860]due to the addition of these water samples,
[00:06:24.670]and then not as large of a signal suppression
[00:06:26.730]by the uranium addition.
[00:06:28.400]We strongly suspect that this is due
[00:06:30.390]to the phosphorus present in these samples
[00:06:32.170]interfering with the probe.
[00:06:34.160]We have successfully demonstrated
[00:06:35.610]the use of the lysine six probe
[00:06:37.310]as an electrochemical peptide-based uranium sensor.
[00:06:40.840]This probe can interact with uranium
[00:06:42.560]in a concentration-dependent manner,
[00:06:44.410]and thus could be used to quantify the amount of uranium.
[00:06:47.440]However, some issues still need to be resolved.
[00:06:49.960]The most important challenge is to identify
[00:06:51.790]the interfering species,
[00:06:52.920]and find a method to remove them from real water samples
[00:06:55.740]to allow the system to be used in the real world.
[00:06:58.660]In addition, although this probe
[00:07:00.160]can be regenerated in 1% SDS,
[00:07:02.700]it is not reusable after the regeneration process,
[00:07:05.350]and it is necessary to find
[00:07:06.700]an improved sensor regeneration method
[00:07:09.260]to regain the usability.
[00:07:11.070]Lastly, gold-plated screen-printed electrodes
[00:07:13.790]will also be used to fabricate the K6 sensor,
[00:07:16.490]as these are more cost-effective.
[00:07:19.420]This work was supported
[00:07:20.480]by the National Science Foundation REU program,
[00:07:23.160]University of Nebraska REU program,
[00:07:25.420]Nebraska MRSEC, and NCIBC.
[00:07:29.540]Thanks for listening.

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Uranium (VI) Ion Detection Using a Hexalysine Probe

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