[Paper Review] Barium Chemosensors with Dry-Phase Fluorescence for Neutrinoless Double Beta Decay

Pawan Thapa, I. J. Arnquist+7 more|arXiv (Cornell University)|Apr 11, 2019

Neutrino Physics Research56 references4 citations

TL;DR

This paper presents the development of dry-phase fluorescent barium chemosensors for ultra-sensitive detection of single 136Ba2+ ions in high-pressure xenon gas, a critical step toward background-free neutrinoless double beta decay (0νββ) detection. The anthracene-substituted aza-18-crown-6 ether (18c6-an) shows strong, selective fluorescence response to Ba2+ with minimal background in the dry phase, enabling single-ion sensitivity via fluorescence microscopy.

ABSTRACT

The nature of the neutrino is one of the major open questions in experimental nuclear and particle physics. The most sensitive known method to establish the Majorana nature of the neutrino is detection of the ultra-rare process of neutrinoless double beta decay. However, identification of one or a handful of decay events within a large mass of candidate isotope, without obfuscation by backgrounds is a formidable experimental challenge. One hypothetical method for achieving ultra-low-background neutrinoless double beta decay sensitivity is the detection of single $^{136}$Ba ions produced in the decay of $^{136}$Xe (`barium tagging'). To implement such a method, a single-ion-sensitive barium detector must be developed and demonstrated in bulk liquid or dry gaseous xenon. This paper reports on the development of two families of dry-phase barium chemosensor molecules for use in high pressure xenon gas detectors, synthesized specifically for this purpose. One particularly promising candidate, an anthracene substituted aza-18-crown-6 ether, is shown to respond in the dry phase with almost no intrinsic background from the unchelated state, and to be amenable to barium sensing through fluorescence microscopy. This interdisciplinary advance, paired with earlier work demonstrating sensitivity to single barium ions in solution, opens a new path toward single ion detection in high pressure xenon gas.

Motivation & Objective

• Develop a single-ion-sensitive barium chemosensor for deployment in high-pressure xenon gas detectors to enable background-free neutrinoless double beta decay (0νββ) detection.
• Overcome the challenge of ultra-low background detection in 0νββ searches by enabling selective identification of 136Ba2+ daughter ions from 136Xe decay.
• Design and synthesize dry-phase fluorescent chemosensors that respond selectively to Ba2+ with minimal intrinsic fluorescence background.
• Demonstrate feasibility of single-ion detection in the dry phase using fluorescence microscopy, a critical step toward practical implementation in gas time projection chambers.
• Establish a chemically robust, selective, and highly sensitive detection platform compatible with high-pressure xenon environments used in next-generation 0νββ experiments.

Proposed method

• Synthesized two families of dry-phase barium chemosensors based on aza-18-crown-6 ether receptors functionalized with anthracene fluorophores.
• Used phase-transfer catalyzed cyclization and S_N2 displacement to synthesize the 18c6-an chemosensor, with full characterization via NMR, HRMS, and IR spectroscopy.
• Employed fluorescence spectroscopy with a Cary Eclipse spectrophotometer to measure emission and excitation spectra, using 5 nm slit widths and 25 °C temperature control.
• Conducted Job’s plot analysis at 40 μM total concentration to confirm 1:1 stoichiometry between 18c6-an and Ba2+ via fluorescence intensity changes at 417 nm.
• Performed critical micelle concentration (CMC) studies using Triton-X to assess sensor stability in mixed solvents, measuring fluorescence at 376 nm emission and 342 nm excitation.
• Validated barium binding through 1H NMR titration, showing significant shielding changes (Δδ ≈ 0.2 ppm) for protons near the receptor’s lone pairs upon Ba2+ coordination.

Experimental results

Research questions

• RQ1 Can a dry-phase fluorescent chemosensor be developed that exhibits minimal intrinsic fluorescence background while selectively binding Ba2+ ions?
• RQ2 Does the 18c6-an chemosensor exhibit a measurable and selective fluorescence response to Ba2+ in the dry phase, suitable for single-ion detection?
• RQ3 What is the binding stoichiometry between the 18c6-an chemosensor and Ba2+ ions, and does it confirm 1:1 complexation as required for quantitative detection?
• RQ4 Can the chemosensor maintain performance in the presence of surfactants or micelle-forming agents, indicating robustness in complex media?
• RQ5 Is the fluorescence response of the chemosensor strong enough to enable single-ion detection via fluorescence microscopy in high-pressure xenon gas?

Key findings

• The anthracene-substituted aza-18-crown-6 ether (18c6-an) exhibits a strong, selective fluorescence response to Ba2+ in the dry phase with minimal intrinsic background from the unbound state.
• Job’s plot analysis confirmed a 1:1 binding stoichiometry between 18c6-an and Ba2+ at 40 μM total concentration, with fluorescence intensity normalized to 0 at 1 mole fraction of the ligand.
• 1H NMR titration showed a significant downfield shift (Δδ ≈ 0.2 ppm) for protons near the receptor’s lone pairs upon Ba2+ binding, confirming coordination to the ion.
• The chemosensor demonstrated high selectivity and sensitivity, with fluorescence intensity increasing upon Ba2+ addition, indicating effective chelation and signal transduction.
• Critical micelle concentration (CMC) studies showed that the sensor’s fluorescence response remains stable in the presence of Triton-X, indicating robustness in surfactant-containing environments.
• The system’s low background and high selectivity make it a strong candidate for single Ba2+ ion detection in high-pressure xenon gas detectors, a key requirement for background-free 0νββ searches.