Introduction: Mass spectrometry imaging (MSI) is quickly gaining popularity in the field to create chemical photographs of localizations within tissues of both plants and animals. Unfortunately, it is unfeasible for everyone interested in MSI to own a high resolution MS for the task to obtain the high levels of spatial resolution necessary. Therefore, we aim to utilize graphene as a co-matrix to enhance both spatial resolution and ionization efficiency.
Objectives: The primary objective of this study was to increase the spatial resolution and promote higher ionization in a MALDI imaging experiment through the utilization of monolayer graphene in tandem with traditional MALDI matrices.
Methods: To test the ability of graphene to enhance MSI, brain slices were harvested from control Sprague-Dawley rats to be utilized in imaging trials. Brain sections were cryosectioned at 12 μm thickness and thaw mounted onto conductive ITO coated slides. Monolayer graphene on copper was transferred to 120°C thermal tape to be utilized in the study. Half of the brain slice had a monolayer of 2D graphene (1cm x 1cm) deposited over the top half-section of the sample. Samples were then placed into an in-house sublimation chamber and were coated with either 1,5- Diaminonapthalene (DAN), 2,5- dihydroxybenzoic acid (DHB), or ɑ-cyano-4-hydroxycinnamic acid (CHCA). Samples were run utilizing a 12T FT-ICR MS using MALDI in positive ionization mode. Samples were acquired at 200-μm spatial resolution. Analyses were performed utilizing a combination of SCiLS lab, Metaspace2020, and Cardinal MSI.
Results: The first study to test the capabilities of graphene was done in conjunction with DAN. Initial results from this data set saw higher intensity signals observed within the portion of brain tissue with a 2D graphene monolayer transfixed prior to sublimation. In particular, an enhancement of species from the motor cortex region of the brain is observed. A number of lipids exhibit a 5 to 10-fold enhancement in ion signal with the combination of DAN and 2D graphene. This is in accordance with previous studies in neat samples deposited onto 2D graphene.
Through the development of sublimation techniques for CHCA and DHB in tandem with wash techniques, the enhancement of lower abundance species such as neuropeptides will be reported.
Conclusion: The utilization of graphene to enhance resolution and ionization in MALDI MSI proves to be promising. The results presented herein warrant further investigation into different tissue types to gain an understanding of the capabilities of graphene.