Add rel=”nofollow” to a link in a Button

Hi,

How do I add a nofollow tag to a link when using the button shortcode?

[button href="http://onlinediamondbuyingguide.com/gallery/" css_classes="button_hilite _button"]View More Pictures…[/button]

I tried a few variations such as this but to no avail.

[button href="http://onlinediamondbuyingguide.com/gallery/" rel="nofollow" target="_blank" css_classes="button_hilite _button"]View More Pictures…[/button]

Could you advice?

paul